You can place Vines into Vines

Well, you can also place any other block in the vine to quickly overwrite it.
But losing vines by replacing vines by vines doesn't seem right at all

This is not a bug. You can do the same with ladders and signs.

Attempting to place a ladder on a ladder will make it attach to the wall, signs float. When placing a vine on a block that already has a vine it places the vine on the vine block, much like placing water on a block that already has water.

Cannot reproduce with 13w02b.

Seems to be fixed.