Judging from WingsOfStardust's posts, ox abandonment seems to be a problem in Dvergheim as well. Something minor that might help is to add a "weighty," more-rewarding quest for the return trip from each place.
The cheese can be carried without an ox, and as far as I know, there are no Dvergheim to Vallaki quests at all.
That's what pushes players towards "one-way trips."
(To be sure, having a place to sell oxen would be convenient for players, but it rewards them for doing nothing. IRL, this would not solve the problem but make it worse as Vallaki's oxen population would shrink and the two other villages would still have an over-abundance.)