The general problem is under investigation, my answered but not yet resolved ticket is #7136. For 1+1=2 vanished vans in two towns it was a counting bug. In one case 40 vans in town offers were counted as 80, and the system confiscated 1 of allegedly 110 vans with a notification (trade notes). In another town I also got a notification that 1 of allegedly 71 vans was "homeless" and removed. Both towns had 70 vans.
Some hours later 20 additionally vans in the first "110" town vanished without a notification. Today I needed these 20 missing vans, and built 20 new vans -- fortunately this town was not busy to build a trader.
Update: one van returned, costs for 20 vans paid back, ticket closed, thanks. The last van in another town also exists again.
Edited by dunnoob - 12 Aug 2012 at 00:29