A key flaw with Lapras is that it comes later than other Water-types one might want for their team. In addition, it has to catch up in levels, which is worsened by its Slow growth rate.
Vaporeon and Lapras arrive at the exact same level, even though you can get Vaporeon significantly earlier in the game. Vaporeon's 110 special attack is a lot better than Lapras's 85. Lapras also has a fighting weakness, which can be a problem sometimes. I'd pick Vaporeon over Lapras, even though Lapras does have the better offensive typing and moves.
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Here's a moveset.
Vaporeon @ leftovers/mystic water
EVs: 100 Spe
- surf
- ice beam
- bite/acid armor/waterfall
- bite/acid armor/waterfall