Turns out it was not a problem with screep positions at all but code that was run only when screen positions were within a certain negagative range. The only time the positions could be in that range was when multiple monitors were involved allowing the screen to be moved such that its left and top were both in the high negative numbers. The error itself was refering to the _VFP object when it should have been addressing the _SCREEN object.

Thanks for the advice.