I think I have a problem...

try loosening it up with some vaseline or vegetable oil

Sounds like pasty butt.  This will kill your chick.  You need to moisten the dried stuff on his bottom as soon as you can.  Use some warm water or soapy water.  This should help you remove the pasted up stuff and prevent any real pain to the chick.  Once everything is removed, dry him off as much as you can and make sure he is kept warm until he completely dries.  Search on here for pasty butt if you want more advice.

.....or a wet rag.   Then maybe put vaseline or vegetable oil on the skin so the poo won't stick in the first place?   Not sure if it would work.

I am not an expert by any means and never dealt with this but I read about the dried poop and saw that using a paper towel with warm water would help in removing it gently.  Also, you should address this soon as this may cause your chick to die as it blocks its vents.

Hope this helps.

I like the vaseline/oil idea, too. I have also held them on their side, vent under a very light stream of skin temp water, this will loosen it up, but be sure to pat dry and put back in a heated brooder ASAP. Poop will stick sometimes, and it is usually only a problem if it covers the vent.

I hope it pulls through.

I had the same issue. It was really bad. I took a little bowl of warm water and stuck the back end in, let it soak and gently wiped it with a paper towel. Make sure not to let the little one get cold. Dry the chick once back end is clean and apply Vaseline or oil. I used olive oil because that's all I had. I made sure to put fresh olive oil on with a q-tip everyday until his/her poops are normal.

Oh if you soak the back end and the chick is possibly a little constipated which I didn't realize mine was. I ended up needing a second bowl because the warm water made my little one poop like crazy. I could see the relief on Nugget's face afterward though.

wipe the olive oil on the vent, took care of the problem for me.