vfl is strappable and stable to like .25 ohms at 18v. the viper wont like 1 ohm and iirc is not strappable. and it wont go over 15v (i woulnt try it at least). the terminals are those gay 2x4awg and they are too small for 4 guage. more like 6 guage. and they are the removeable ones. the VFL has set 0awg inputs and 8awg speaker terminals.
id take the vfl. ive used both. i like both amps, especially if you have something like a TC LMT that only does power at 3 ohms. then something liek the viper that does 2500w@2 is nessicary. otherwise, stick with a VFL. both are real powerful amps however.