I think the biggest thing that doesn't sit doesn't feel right to me is the changing hp for the vines. If the point is that you are wrapped up by multiple vines, shouldn't you just get tangled again as you try to cut out? Or maybe even burst out with a Str check?
Well if we followed the original version the vines hit points represent all the vines in a square, so once you cut a creature free the strangler vine can't attack them in that square.
While that seems OK for a hazard, it doesn't feel quite right for a creature.
My rationale for having larger patches have more hp was that they'd have more and longer vines to hold a victim, an argument I think still holds water.
However, I agree that it should just add new vines while a victim is trying to cut themselves free, so how about we say the damage has to be done in one round, so it doesn't have time to add more tendrils?
How about the following:
Strangling Vines (Ex): Any creature that enters the space of a strangle vine must succeed at a DC10 Reflex save or be seized by its vines. If they succeed, they must check again every round they remain within the plant's space.
Seized creatures are entangled, cannot move out of the space they where standing in when the vines seized them, and take 1d4 damage (plus Strength bonus, if any) per round from being "strangled". Any creature in a strangle vine's grasp cannot speak or cast spells with verbal components, because the vines wrap round their head and neck and try forcing themselves down their victim's throat.
A seized creature can escape by succeeding at a DC14 Strength check or a DC14 Escape Artist check. They can also break free by killing the strangle vine or cutting the vines that hold them;
an opponent must do at least 4 points of damage to the AC 13 vines in a single round to free a seized creature (8 hp for Large strangle vines, 12 hp for Huge, 16 hp for Gargantuan and 20 hp for Colossal). The damage must be done over a single round because the strangle vine will wraps fresh tendrils around the seized victim the next round to replace any that have been severed or crushed. Damage done to the vines is subtracted from the hit points of the entire plant. The DCs are Strength-based and include a +4 racial bonus on the Strength and Escape Artist checks.