Yes, I prefer the standard grapple check to climb out (vs Climb checks), the silly elastic action to close the hole, etc. 
 
		 
Okay then, let's use that and get on with the stats.
I cut down the DR from a treant's 10/slashing to 5/slashing, since I don't think these things should be as tough as a walking oak tree.
It's all pretty straightforward now apart from the question of how much damage it does with its "whip". Do we treat it like an actual whip [1d4+7 nonlethal?], regular damage like a tendriculos's tendril [1d4+7?], or a no-damage grapple like the original [special?].
Pitcher Plant, Giant
Large Plant
Hit Dice: 5d8+20 (42 hp)
Initiative: -2
Speed: 0 ft. (0 squares)
Armor Class: 14 (-1 size, -2 Dex, +7 natural) touch 7, flat-footed 14
Base Attack/Grapple: +3/+12
Attack: Tendril +7 melee (1d4+5)
Full Attack: Tendril +7 melee (1d4+5)
Space/Reach: 10 ft./0 ft. (20 ft. with tendril)
Special Attacks: Drowning, improved grab, swallow whole
Special Qualities: Blind, damage reduction 5/slashing, tremorsense 20 ft., plant traits
Saves: Fort +8, Ref -1, Will +1
Abilities: Str 21, Dex 6, Con 18, Int -, Wis 11, Cha 3
Skills: —
Feats: Blind-Fight (B)
Environment: 
Any?
Organization: Solitary or cluster (2–8)
Challenge Rating: x
Treasure: 
10% coins?
Alignment: Always neutral
Advancement: 6-8 HD (Large); 9-15 HD (Huge) [
based on Tendriculos]
Level Adjustment: —
Combat
Blind: Giant pitcher plants are immune to gaze attacks, visual effects, illusions, and other attack forms that rely  on sight.
Improved Grab (Ex): To use this ability, a giant pitcher plant must hit a creature at least one size  smaller than itself with its tendril attack. It can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can try to swallow the opponent in  the following round.
Swallow Whole/Drowning (Ex): A giant pitcher plant can try to  swallow a grabbed  opponent by making a  successful grapple check. The  interior of the plant is a vase filled with digestive fluids, each round  the opponent takes 1d4 points  of acid damage and must succeed on a   DC15 Swim check or sink under the fluid and risk drowning. A swallowed  creature can climb out of the vase with a successful grapple  check.  This returns  it to the plant’s maw, where another successful grapple  check is needed  to get free. The inside of the vase is lined with  needle-sharp spines that do 2d4 piercing damage to the opponent for  every grapple attempt they make to escape. A swallowed creature can also  cut its way out by using a  light slashing or piercing weapon to deal  25 points of damage to the giant pitcher plant's interior (same AC and  DR as the plant's exterior). Once the creature exits, elastic action  closes the hole; another  swallowed opponent must cut its own way out. A  Large giant pitcher plant's interior can hold 2 Medium, 8Small , 32  Tiny, 128 Diminutive or 512 Fine opponents.