Depends on what you want to use it for. I have a VF-16 that I use to record my own music, I'll be recording my band with it real soon, I also use it to make pracice cd's for myself. In this respect it works out great, for the price I'm not complaining about anything. If you are using for home studio type things a Vf-16 should work fine, I'm sure it would work good in pro-recording conditions too, in certain circumstances.
As far as the 1680 goes, what version is it? Does it come with BOTH effects cards, this is a big question as they are EXPENSIVE if you have to get them. If it's a newer 1680 with both the effects cards I would have to seriously consider it if I were you. If it's an older 1680, don't even consider it imo.
The VF has access to all 16 faders at one time, 1680 doesn't.
The 1680 comes with Mic and amp simulations (if it has effects cards), the VF doesn't
The new VF will come with a warranty, the 1680 will not.
Etc.....
Just depends on which of these factors are important to you.
I don't think you'll notice much of a differance sonically between the two. Just go for the one with the features you'll be happy with.
One other important factor, is 16 bit good enough for you??
sounds fine to me but a lot of people are opting to go with 24 bit as it will soon be the new standard. That doesn't mean much to me but it might to you.
Hope this long reply helps.

Phil W.
Oh yeah, If you get something new, no matter where you get a price from get online with someone from Full Compass, they'll usually beat it. Woulda saved almost $100.00 on my VF if I'd known about this at the time. Good Luck!