Just my two bits, but I don't think its reasonable to expect vendors to comment on your pictures just because you gave them credit.   Think of it this way.  Suppose you were a vendor and you made just one item.  Suppose that sold 200 copies.  Now suppose out of that there were 100 people posting images using your item... you'd spend all your time posting c&p thank you notes.   Personally, I'd rather those vendors focus their creative energy on creating more content than trying to come up with creative ways to say thank you to the 1000th person to use their product in an image.

Its nice if they do happen to drop by and post a comment.  Then again its nice if ANYONE posts a nice comment.  Even better if they post a thoughtful comment about what they particularly liked about the image.

As for why I include vendor credits, when I remember to do so (and I admit I don't always do it, I get a lil absentminded... ) its more a courtesy to the vendor and to the visitors.  For the vendors it gives them a little free advertising which helps them along and helps keep that content rolling out.  For the visitors it makes it easy for them to find items I used they might want... sometimes you get a thank you just for that.   Mostly I do it just to be nice, its not about whether I get anything out of it.

But that's just my opinion on it.