In the vale of shadows, I found the sanctum key in one of the outer tombs. Then I went to Kresselack's tomb. I went up to the sanctum door, but all I get is 'locked.' I've moved the key around, used different people to try and open it, and even added another sanctum key through Dalekeeper. I still can't get in.

You need several different keys, and you need to collect them from various tombs in the Vale.

I have the proper key according to the walkthoughs I read to figure out how to get past that damned door. I think the game glitched on me.

Have you explored everwhere else in K's tomb and elsewhere in the Vale?

yup

I have some vague recollection about having the same problem. I think you need to pull some lever(s) first.

In the room with the triagle tomb in the middle, near the front is a pillar, if you look closly you should be able to find the lever you need to pull to open the door.

However to use the lever you need to get the WOLF talisman from the room in the south coridor