take damage over time on lash instead of recharge speed, with a pirahna x there's only a .20 difference which i don't think you'll notice and i think fast recharge is better than shield penetration but that's up to you
If you use the second build, go with DOT instead of Recharge Speed on Rank 5 of Lash. Also, I would highly recommend taking Bonus Power on Rank 6 of Biotic Charge instead of Barriers, so that you have the potential to pull off Smash immediately after gaining the 40% power damage boost.
Personally, I like the first build better, but the second one is good too. Lash is one of those powers that's very useful on a few different enemies (Phantoms, Combat Engineers, Collector Captains, and Geth Rocket Troopers to name a few), but it's even better on the Adept since he has Singularity to prime targets with. The Vanguard doesn't, so I feel that it loses a bit of usability.