Is there any reason why a ported enclosure should not exceed the VAS of the driver by volume? Also, is there any reason why you wouldn't want to , if possible, tune this same enclosure below the FS of the driver to extend the low end?
Vas is merely a technical specification, not an enclosure requirement, even though they have the same units (volume).
Vas is used in conjunction with the other T/S parameters to form a complete picture of frequency response (predicted). Alone, the only thing this parameter tells you is only what it specifies: a volume of air having the same compliance when acted upon by a piston of the same Sd as a particular speaker---nothing more, nothing less.
One thing you could apply across the board in a very broad generalization, is that the larger the enclosure is, the lower the mechanical power handling of the speaker.
As far as tuning, depending on the other T/S parameters tuning too much below Fs could give you an excursion maximum just above Fb (the tuning frequency) that could be too high for the power requirements/input. It all depends on the final enclosure alignment and the actual speaker in question. Enclosure modeling via WinISD, Horn Response, Akabak, etc would give you actual insight into all these things without having to rely on broad generalizations.