Guess it doesn't like comparing a list to a list.
This works thou:
if (llList2Vector(llGetPrimitiveParams([PRIM_SIZE]),0) == <1.564000, 1.000000, 0.080000>
EDIT: I THINK what is happening is that comparing the vectors in the lists is actually comparing NULL_VECTOR to NULL_VECTOR:
See this wiki entry:
"Q: Why are there so many llList2* functions?
A: LSL2 doesn't support run-time typing, meaning you have to actually specify the type of each variable. The Lindens could have implemented a single function, llList2String, then required explicit typecasting, as in (integer)llList2String(foo, 0). However, that isn't as memory- or CPU-efficient as llList2Integer(foo,0) (yes, really) and the amount of work required for the Lindens to implement seperate functions for each type was negligible.
You can still typecast values stored in a list, of course, but it's advised to use the llList2* functions, unless it's a vector or rotation stored as a string, in which case you should cast to a vector or rotation after using llList2String. llList2Vector and llList2Rotation do not cast from strings automatically. See the next question for more.
Q: Have the old problems concerning the odd behavior of llList2Vector and llList2Rot when requesting a TYPE_STRING element from the list been fixed yet?
A: No. If a list element is a string containing text that would otherwise be directly typecast to a vector or rotation, It will not be converted. The resulting vector or rotation will be ZERO_VECTOR or ZERO_ROTATION respectively. This is a bug. To get around it, use llList2String to retrieve elements from your list, then cast them to vectors or rotations as needed: (vector)llList2String(positions,2);"
While not directly addressing your problem, I think an offshoot of it is happening to you.
