I have this exact same problem on my 86 vanagan.

My old vbelt that worked for 20+ years recently fell of and broke. Then I
bought new ones, so far three, and they keep falling off after driving 10
minutes to 8 hours.

I found out that the old vbelt is size 9.5x1080 (which is also the saze
stated in the Bentley manual) while the only new ones I can find are
10x1075. It looks like the old original vbelt had a better fit, i sat deeper
in the track.

Could this be the problem, and if yes, does anyone who know where to get the
REAL ORIGINAL SIZE 9.5x1080?

Best
Stefan