The SW API help says that if it's associated to a document, it will return a null value - based on what you wrote, if GetEntityState returns false, that could mean that it's associated to a document. "If the attribute is not associated to any entity (that is, it is associated to the document), all queries return false."
The other suggestions is that it's invalid or suppressed
Along the lines William said... A sketch point is not an entity. So, (not knowing the rest of your code), I would conclude that your attribute is not attached to an entity.
From the help:
I don't see sketchpoint in that list of what things are entities.
If you have a Face2, you can set it to an Entity, like
Set swEnt = swFace
Try the same with a SketchPoint and you get a type mismatch.
Yep - you're both correct. In my haste I read "vertex" as "point". Looks like I'll need to go down the persistent ID route link Josh included.