If Vaporeon is soloable, how couldn't Lapras?
Lapras has a lower base attack stat than Vaporeon, but it has a higher base defense stat. They have the exactly same base stamina stats.
Having a 3 additional base defense stat isn't going to make much difference in battles.
Vaporeon has only 2 weaknesses, while Lapras has 4 weaknesses. Though, Lapras has an access to a better defensive quick move than Water Gun, named Ice Shard.
Six level 30 Machamps should be able to defeat the Lapras in the time limit.
However, if Vaporeon isn't soloable, nor does Lapras. The most important thing we care about the raid battles is the DPS. And Lapras has a greater tankiness than Vaporeon.
You can usually get replaced by another Pokémon in your lineup when one faints, but you will have to start all over again if you run out of time.
Answers
Celandro from pokebattler estimates that lvl 3 get 2000 extra stamina, while lvl 4 get 8000 (eight) extra.
https://articles.pokebattler.com/2017/06/25/raid-research-update/
Raid tier 1 – Bonus Stamina 0
Raid tier 2 – Bonus Stamina 315
Raid tier 3 – Bonus Stamina 2000
Raid tier 4 – Bonus Stamina 8000
The above also explain why Alakazam and Jolteon are the toughest to crack from lvl 3 bosses: deal a lot of damage because of their attack and quick moves, lack of stamina not an issue anymore. And on top of that lack of maxed power up counters in player inventories :).
