I see what you're saying now, but it's not a standard severing mechanic, and it doesn't seem crystal clear from the write-up. The problem is, when I think about how to clean this up, I'm led toward making each attached vine the result of a touch attack or something. The other thing that bothers me a little about this is that it's too much advancement information. That kind of stuff is usually left to the DM to figure out.
Well it's easy enough to make clear that there can be more than one vine holding them. Upon reflection, using the "half damage when severed" rule makes little sense, since it has multiple vines per square. I'd change it to the plant takes damage on a 1:2 ratio.
Also, in the original version once a square is "cleared" the strangle vine can't attack in it - should we include that trait in the Strangling Vines?
Something like "If all the vines in a square are severed the plant can not seize or strangle creatures in that square." I would rather not, since (a) I like to think of the vines as being slightly mobile, allowing them to slither into empty squares, and (b) it'd mean you'd have to keep track of its HPs per square which seems a
little fiddly.
Revising...
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
through all the vines that hold them; the vines are AC13 and have 4 hit points (8 hp for Medium strangle vines, 12 hp for Large, 18 hp for Huge, 24 hp for Gargantuan and 30 hp for Colossal).
The main plant takes 1 point of damage for every 2 points of damage done to its vines. [If all the vines in a square are severed the plant can not seize or strangle creatures in that square. - don't much care for this option]
The DCs are Strength-based and include a +4 racial bonus on the Strength and Escape Artist checks.
