Possibilities abound. For vocals, some love the MXL V67G (at about $99.). I love
my Studio Projects B1's (same price, including the shock mount). The SM57 needs more gain than many others, but it's considered one of the best all-around mics ever made.
The ones I mentioned are condensor mics, and so, unlike the SM57, you'll need to use the phantom power on the VF160. I don't think you'll have to boost the gain up as much. The B1's are VERY quiet, but like most condensors, they pick up EVERYTHING... your neighbors, your breathing, the hard-drive whirring, etc. You'll also need a pop-shield to tame the popping p's. Still, nice condensors sound great on vocals. You really should have at least one.
Although I've had good experience with the pre's on my VF80, you might consider an external pre. There are ones like the M-Audio DMP3 (2channel @ $199.). I have the Studio Projects VTB1 (1channel @ $129.). The DMP3 has a great reputation for nice clean sound. As for the VTB1, you can boost the gain up all the way WITHOUT adding any noise. It runs about -129db s/n. VERY QUIET, plus you can dial in a tube circuit for warmth (or distortion). On the 'SS' postion however, the tube circuit is entirely bypassed... With either of these externals, you may notice the most improvement on your SM57 because you'll be giving it all the boost it requires, without adding any preamp circuit noise.
Just remember, when you use the phantom power on an external preamp, don't use the one on the VF160...