To summarize:

There are only 2 conditions in which I am able to get vartype to generate an error:

1. _vfp.foo  [or _vfp.anything not defined to _vfp]

2. oForm = newobject("form")  && or pick any object to create
   vartype(oForm.BadProp) returns U
   oForm = .null.
   vartype(oForm) returns X
   vartype(oForm.anything) generates an error

Personally, I see no problem with the generation of the error for the second condition. I am ambivalent about the first condition because I seldom use _vfp. If I do, I always make dang sure the property exists before I add it to my code.