Can you judge a guitar's (electric) sound merely with a practice amp? (read on...)?

Try playing your guitar through some other amps (friends, stores, etc.) If you can consistently get the sound from other amps that you can't get from your Vox, then the problem's your amp. If you can't quite get the sound you're looking for through other amps, then maybe the problem's your guitar. It also wouldn't hurt to have a qualified tech give your guitar a good tuneup.