vaginapagina | Sex toy question

Thanks to everyone who lent advice on my last question (re: STI/STD) testing... I'm going to start with HIV testing, since I've got info on getting it done for free.

Now, I've some more questions on a different topic:

I've got a fair collection of vibrators, and a couple of dildos. Normally I store them all together in a small chest in my room. Amongst these I had one dildo that I believe was vinyl rubber, one jelly, and another that I can't remember what it was made of. The vinyl rubber one is the one I've had problems with. Two problems, specifically:
1) When I first bought it, I had no problem with the sensation/feel of it. Last year a friend and I used it with a harness, using water-based lubricant. I tried using it on an occasion after that (without lube), and it caused a slight burning sensation. I also found that, no matter how many times I washed it, it seemed to end up with a slight greasy feel.
So what the heck happened here? It seemed like it must have had some kind of reaction to the lube, but how/why? I tried using it once more at a later date, and got the same uncomfortable results. Haven't used it since then, obviously.
2) As I said, I store all my toys together. Well, apparently the three dildos ended up resting next to each other in the box... and the vinyl rubber one MELTED the other two. D: Like, not all the way. But there was a small melted spot on the jelly, and the other one had a LARGE melted area on the side. I ended up having to throw it out, which makes me sad because it was one of my very favorites ;_; There's no damage at all to the vinyl one at all.
Could this have something to do with question #1 (the lube), or should I just not store toys made of different materials together?

ALSO - - another question. I've often heard of people using condoms with their sex toys (when using them with a partner). Why is this done?
Generally, the only person "receiving" my toys is me, so I never really worried about it, but I figured this would be good to know about for future reference.