Some of the calculations here aren't exactly what you'd see in battle, but rather "what-if" situations if 28 was important enough to warrant building against.
Physical Attacks
252 Atk Zeraora Knock Off (97.5 BP) vs. 0 HP / 0 Def Vibrava: 115-136 (46.5 - 55%) -- 66.8% chance to 2HKO
252 Atk Zeraora Knock Off vs. 0 HP / 0 Def Vibrava: 77-91 (31.1 - 36.8%) -- 72.7% chance to 3HKO
252 Atk Zeraora Plasma Fists vs. 0 HP / 0 Def Vibrava: 89-105 (36 - 42.5%) -- guaranteed 3HKO
252 Atk Zeraora Play Rough vs. 0 HP / 0 Def Vibrava: 214-252 (86.6 - 102%) -- 12.5% chance to OHKO
252+ Atk Choice Band Rillaboom Wood Hammer vs. 0 HP / 0 Def Vibrava in Grassy Terrain: 122-144 (49.3 - 58.2%) -- 66.4% chance to 2HKO after Grassy Terrain recovery
252+ Atk Choice Band Rillaboom Knock Off (97.5 BP) vs. 0 HP / 0 Def Vibrava: 204-240 (82.5 - 97.1%) -- guaranteed 2HKO after Grassy Terrain recovery
252+ Atk Choice Band Rillaboom Grassy Glide vs. 0 HP / 0 Def Vibrava in Grassy Terrain: 71-84 (28.7 - 34%) -- 99.4% chance to 4HKO after Grassy Terrain recovery
252+ Atk Choice Band Rillaboom U-turn vs. 0 HP / 0 Def Vibrava: 147-174 (59.5 - 70.4%) -- guaranteed 2HKO after Grassy Terrain recovery
8 Atk Hippowdon Earthquake vs. 0 HP / 0 Def Vibrava: 72-85 (29.1 - 34.4%) -- 100% chance to 3HKO after sandstorm damage
Blissey Seismic Toss vs. 0 HP Vibrava: 100-100 (40.4 - 40.4%) -- guaranteed 3HKO
Blissey Seismic Toss vs. 216 HP Vibrava: 100-100 (33.2 - 33.2%) -- guaranteed 4HKO
Misc. calculations (mostly in-theory and some other things)
252+ Def Kommo-o Body Press vs. 0 HP / 0 Def Vibrava: 84-99 (34 - 40%) -- guaranteed 3HKO
252 Atk Excadrill Earthquake vs. 0 HP / 0 Def Vibrava: 102-120 (41.2 - 48.5%) -- guaranteed 3HKO
252 Atk Excadrill Rock Slide vs. 0 HP / 0 Def Vibrava: 204-240 (82.5 - 97.1%) -- guaranteed 2HKO
252+ Atk Mandibuzz Foul Play vs. 0 HP / 0 Def Vibrava: 214-253 (86.6 - 102.4%) -- 18.8% chance to OHKO
252 Atk Choice Band Syclant U-turn vs. 0 HP / 0 Def Vibrava: 190-225 (76.9 - 91%) -- guaranteed 2HKO
0 Atk Toxapex Knock Off (97.5 BP) vs. 0 HP / 0 Def Vibrava: 58-69 (23.4 - 27.9%) -- 85.8% chance to 4HKO
+6 252+ Atk Huge Power Azumarill Aqua Jet vs. 0 HP / 0 Def Vibrava: 191-225 (77.3 - 91%) -- guaranteed 2HKO
Although we can't take a Play Rough with 100% certainty, the rest of Zera's attacks can be received somewhat comfortably. We live 2 Plasma Fists, a Plasma Fists > Knock Off, and Knock Off > Knock Off. Rillaboom can 2HKO with Wood Hammer, but has no real way of scaring us with any other Grass move, and we have just enough bulk to live boosted Knock Off. Hippo can't 2HKO us, and Blissey at best 3HKOs with Toss. All of these except Zeraora don't factor in HP we can recover from Leech Life, which substantially raises our longevity (or probably KOs you, if you're Rillaboom).
The other attacks are a selection of moves we are able to take, including max power Azumarill's Aqua Jet, which is cool but only helpful if it's low enough to actually fall to any of our moves. Max Def Kommo's Body Press is mentioned as it's the strongest unboosted Body Press in the game at the moment, and I figure that's an attack we'll take fairly often from some Pokemon.
I don't think I need to mention it, but any decently strong SE or nuclear neutral hit on our physical side is going to down us. This includes anything from a 0 Attack Dragapult Dragon Darts to Banded Urshifu Wicked Blow.
Special Attacks
252 SpA Zeraora Thunderbolt vs. 0 HP / 0 SpD Vibrava: 44-52 (17.8 - 21%) -- possible 5HKO
252 SpA Zeraora Volt Switch vs. 0 HP / 0 SpD Vibrava: 34-41 (13.7 - 16.5%) -- possible 7HKO
8 SpA Slowking Future Sight vs. 0 HP / 0 SpD Vibrava: 93-109 (37.6 - 44.1%) -- guaranteed 3HKO
8 SpA Slowking Scald vs. 0 HP / 0 SpD Vibrava: 30-36 (12.1 - 14.5%) -- possible 7HKO
8 SpA Slowking Ice Beam vs. 0 HP / 0 SpD Vibrava: 92-110 (37.2 - 44.5%) -- guaranteed 3HKO
0 SpA Rotom-Heat Overheat vs. 0 HP / 0 SpD Vibrava: 102-121 (41.2 - 48.9%) -- guaranteed 3HKO
252 SpA Rotom-Heat Overheat over 2 turns vs. 0 HP / 0 SpD Vibrava: 195-231 (78.9 - 93.5%) -- not a KO
252+ SpA Rotom-Heat Overheat over 2 turns vs. 0 HP / 0 SpD Vibrava: 212-252 (85.8 - 102%) -- 12.5% chance to 2HKO
0 SpA Rotom-Heat Volt Switch vs. 0 HP / 0 SpD Vibrava: 28-33 (11.3 - 13.3%) -- possible 8HKO
252+ SpA Rotom-Heat Volt Switch vs. 0 HP / 24 SpD Vibrava: 38-45 (15.3 - 18.2%) -- possible 6HKO
252+ SpA Equilibra Earth Power vs. 0 HP / 0 SpD Vibrava: 58-69 (23.4 - 27.9%) -- 82.2% chance to 4HKO
252+ SpA Equilibra Doom Desire vs. 0 HP / 0 SpD Vibrava: 180-213 (72.8 - 86.2%) -- guaranteed 2HKO
4 SpA Tomohawk Hurricane vs. 0 HP / 0 SpD Vibrava: 188-224 (76.1 - 90.6%) -- guaranteed 2HKO
252 SpA Tomohawk Hurricane vs. 0 HP / 0 SpD Vibrava: 234-276 (94.7 - 111.7%) -- 62.5% chance to OHKO
252 SpA Life Orb Krilowatt Thunderbolt vs. 0 HP / 0 SpD Vibrava: 49-58 (19.8 - 23.4%) -- possible 5HKO
252 SpA Life Orb Krilowatt Ice Beam vs. 0 HP / 0 SpD Vibrava: 133-159 (53.8 - 64.3%) -- guaranteed 2HKO
252+ SpA Life Orb Krilowatt Thunderbolt vs. 0 HP / 0 SpD Vibrava: 55-65 (22.2 - 26.3%) -- 6.9% chance to 4HKO
252+ SpA Life Orb Krilowatt Ice Beam vs. 0 HP / 0 SpD Vibrava: 146-174 (59.1 - 70.4%) -- guaranteed 2HKO
252 SpA Jumbao Moonblast vs. 0 HP / 0 SpD Vibrava: 212-252 (85.8 - 102%) -- 12.5% chance to OHKO
252 SpA Stratagem Paleo Wave vs. 0 HP / 0 SpD Vibrava: 186-222 (75.3 - 89.8%) -- guaranteed 2HKO
252 SpA Life Orb Stratagem Paleo Wave vs. 0 HP / 0 SpD Vibrava: 242-289 (97.9 - 117%) -- 87.5% chance to OHKO
252 SpA Kommo-o Clanging Scales vs. 0 HP / 0 SpD Vibrava: 212-252 (85.8 - 102%) -- 12.5% chance to OHKO
+2 252 SpA Volcarona Flamethrower vs. 0 HP / 0 SpD Vibrava: 213-252 (86.2 - 102%) -- 12.5% chance to OHKO
28's Special Bulk is the exciting part: we are able to stomach some truly impressive power. Special Zeraora is a complete non-issue if not for Knock Off. The Slows' Future Sight fails to even do over half, and their Scalds do little but fish for burns. Adding Ice Beam onto the set to deal with 28 doesn't help, as it still results in a 3HKO. Rotom-Heat can't 2HKO with two normal-power Overheats, and with neutral nature 252 SpA is unable to KO after two Overheats. Equi doesn't hurt much with EP and we can live a Doom Desire. Tomo's Hurricane can't OHKO without a crit or significant investment. Of course, we're not invincible. Krilowatt scares us with Ice Beam, though we can come in on a resisted Thunderbolt and still take the incoming Ice Beam. Even a theoretical Modest nature set is unable to 2HKO us with this combination. Other Pokemon with SE STAB can scare us, and there's plenty that can take advantage of our defensive typing.
This includes Hydreigon, Kerfluffle, Psyshock Reuinclus, Stratagem, and Kyurem, and of these only Reuniclus is slower. We can try to bolster our Special bulk with investment or an AV, but the returns are not terribly good, and we sacrifice a lot of the positives which come from our item slot and status moves if going the AV route.
The Special Bulk suggested is enough to always live a potential 8 SpA Future Sight > Scald from Slowbro and maintain enough HP to take 1 Seismic Toss from Blissey, assuming you don't get burned in the process. I felt it was important that we can handle them both in the same game if we were to handle them at all.