Ferrying or Bridging ?

[Mr Grumpy]

Is the river you are talking about a one hex wide river or a hexside river?

I suspect the reason your units become isolated is because the ferring enginneers can only supply the river bank hex, this has nothing to do with VST.

If you build a bridge then supply will flow as normal, the time it will take will depend on the river type and the quality of the enginneers.

*FLG beat me to it* :)