I seem to remember placing vines under tripwire/string in past versions, but in 1.12 I am unable to place vines in the space immediately below a tripwire. If you have vines in that space and place tripwire above them, it deletes the vines.

To duplicate:

1. Place a block
2. place string above the space next to the block
3. try to place vines on the sid of the block that has string over it