Tremorsense and earth glide are perfectly acceptable to me.
Updated.
Feats: Alertness, 1 more
Lightning Reflexes? Don't care for that +1 Reflex save much.
Organization: Should we add some (lesser) vilstrak to the mix?
Vilstrak have Gang (2-5) or band (6-20)
I'm seeing band (4-20) on (6-20) in the CC.
Regardless, I think having them mix with regular Vilstrak makes sense. How about one of the following:
Organization #1: Solitary, gang (2-5), band (1-4 plus 3-12 vilstrak) or mob (4-10 plus 5-30 vilstrak)
Organization #2: Solitary, gang (2-5), band (4-20) or mob (2-20 plus 5-40 vilstrak)
I slightly prefer #1, but I'm not that fussed either way.
I'd make it CR3. Their earth glide makes it very difficult to tackle them from a distance, they can just pop out of the rock in the midst of the party's spellcasters and let rip, and if the fight turns against them they can flee the same way.
Besides, their combat capabilities are pretty good, slightly better than the CR3 standard-issue ogre:
SRD Ogre: AC 16, hp 29, greatclub +8 melee (2d8+7) - av. damage 16.
Great Vilstrak: AC 19, hp 30, 2 slams +8 melee (1d6+6) - av. dam 19.
Advancement: 5-x HD (Large)
Well regular vilstrak don't advance in size, being 1-3 HD (Medium), so how about Advancement: 5-12 HD (Large)?
A great vilstrak stands x feet tall and weighs about x pounds. Vilstraks are gray or brown.
I'd make them roughly ogre sized. "A great vilstrak stands 9 feet tall and weighs about 600 pounds."