mental note in favor of non-post-consumer cotton rugs in future...?

We bought a really nice squishy new long rag rug, but unfortunately it has a faint but perceptible mildew stink clinging to it (one I didn't notice until we got it home because it doesn't emerge until my face is right up next to it like lying on the floor close), which maybe I should have expected since it seems to be made of recycled fabric. Mildew seems to be a smell that most people are way less sensitive to than I am - possibly a mild allergy, although it doesn't make my sinuses burn or hurt or anything (YET?). Not fun when frequently attics and basements are full of the smell and nobody else even seems to notice it, but anyway...

I just washed it at 60° with ordinary detergent, which didn't help at all, so I googled it and it seems I should soak it, perhaps as long as overnight, in vinegar water. I have plenty of white vinegar, but we don't own anything big enough to submerge this rug in except the actual washing machine, which is front-loading. So here I go to figure out how to set it to fill and soak things.