the MXL V67 is one of the mics i may put on a vocalist with a thin voice, if it works for the voice and song context. it's got a weird (cloudy? hashy?) low-midrange thing going on, which works on some folks and really interferes on others, and it's got TONS of proximity effect, so you might could use that to your advantage. but it's VERY hit or miss. still, at $99, it's hard not to pull the trigger and see. i figure if it works for a couple vocalists on a couple tracks, then i've spent my $100 well.
the MXL V77 can also do some good things for a thin sounding vocalist, depending on the type of "thinness" in the voice. it's excellent on "mousey female types" who need a little boost. it's a transparent mic, though, and won't really "enhance" much outside of "making what already sounds good sound great".
both of those mics (as well as numerous others) need to be auditioned before i can tell you "which works" on the voice. and even if the mic works on the voice, there's no guarantee that it'll fit the song or production. i've got a number of mics that "work" on my voice as well as a number that "don't", and you'd be surprised how many times i end up using one that "doesn't work" on my voice just b/c it better fits the production.
without hearing the voice, though, i would probably recommend an EV RE20 or a Shure SM7. paired with a decent pre, it's pretty hard to get a bad sound out of either one of those (assuming the vocalist isn't the source of the bad sound

).
cheers,
wade