I've seen a LOT of vendors with dropping Health Bars, and I play in 2D, it seems to be tied to the amount of gold they have on them. Apparently there is a critical limit below which the vendor starts counting down the time until it's out of gold and winds up in the Moving Crate of the house it's in. I tested that theory with one vendor (someone else's) who was selling something expensive I wanted to buy. Before I bought the item, the vendor was nearly red-lined, but afterward it was back to full Health.
Not *certain* that's what's going on, but try giving the vendor more gold and see what happens. It couldn't hurt.
Be well - Pax