I owned a VCI-300 MKii for a very short time–ended up returning it, actually, since I was within the return period–but the reason I returned it didn’t have anything to do with the quality. The jogs were excellent, as were all the controls and the sound output – I never ran into anything like what you’re describing. My problem with it was that it’s a bit limited in what it can do by today’s standards. If all you’re looking to do is beatmatch, mix two tracks together, maybe play around with a couple loops and cue points in a very limited manner, it gets the job done as well as any controller (and pretty cheaply these days, as they’ve been greatly marked down – you can usually find a new one for under $400). But I realized that I wanted a bit more control over creative looping and cutting, as well as effects, and the VCI-300 is a bit dated and sorely lacking in this respect. With the Serato DJ update on the horizon, I just felt like I wanted something with a few more options on board. I have yet to see how much I’m really going to use the capabilities of a more involved controller, but I’d rather at least have the option than to be working with a VCI-300 and not even be able to find out.
Anyway, to answer your question, I think a lot of people are unloading them because they’re a bit dated at this point (it’s effectively a controller from 2008, built for what ITCH could do back then), and with Serato DJ coming, one would likely want a 380 or 400, the Novation Twitch, or any of a number of other ITCH or DJ Intro controllers to really be able to take advantage of what the present and upcoming Serato software is capable of. I honestly doubt that it has anything to do with build quality – again, for the time I was using mine, I really found it to be top-notch where build is concerned. I’d be interested to hear if this is what other people think, though.