The ideal vst would be one which alters, in a fairly random order, pitch, timing & timbre, and only by enough to ensure it simply doesn't sound badly sung whilst avoiding phase problems during playback.

Try Channel Tools! Don't add directly to the vocal track, rather, send the vocal track to a bus with Channel Tools, so you can still have the "dry" vocal in the middle and control width/heft via the send level.
 
Start with the "increase width" preset, and mess with the right channel's delay time to range from fattening/widening (under ~ 25ms) to doubling (over 25ms, but not too much over).
 
(If you ever add Channel Tools directly to a track, make sure the track's Interleave = stereo, otherwise it will have no effect)

One of Dave Pensado's "In the Lair" vids shows him using the Waves Doubler and automating the delay and pitch parameters basically randomly to simulate another vocal take where the doubled part changes pitch a few cents at a time is delayed likewise. Haven't tried it though.

you might try to venture in the way of nudging my friend.
its one of the best kept secrets in audio.

clone the track then either zoom in real close and shift the cloned track a very small amount or use the nudge feature.
try nudge 2 twice and see the results.
no vst needed here. you'll see its like magic and sure to fatten up any skinny vocal

Doing more than one take should always  be the first choice. If thats not an option, try what Charlie said. Its free, u dont have to download anything so, why not?

Seems to me that a lot of people want shortcuts to everything and don't want to put in the time to do it right. 

Beatles recording engineer Geoff Emerick said in a recent interview that he was working on a track for a Sgt Peppers reconstruction recording with the Kaiser Chiefs and they were working on a rhythm track for "Getting Better" and it wasn't sounding too good and the lead guy said that they usually do 3 or 4 takes and the Pro Tools operator sticks it together. Geoff had them do it another 21 times and they got it perfect. Another group called The Magic Numbers (I never heard of either of these groups) had 2 girls singing harmony on "She's Leaving Home" and he told them they weren't starting and finishing the words together. They told him they usually record one vocal at a time and the Pro Tools guy would stick them together. Geoff spent another 5 hours coaching them to get it right and at the end the singers were in tears at how good it sounded. 

There's no shortcut to greatness. 

Well said, Dave!

There's a free VST called ADT (Google it.) But I agree with the 2 Daves - you'll get the best results by actually recording 2 vocals.