I'm curious as to what exactly qualifies as a "safety person" or for that case a "Vessel" when it comes to bag limits for things like Scallops.

The MAF website states:
"Divers may take additional quota for up to a maximum of 2 safety people per vessel."

Does that mean that if I throw my other half in a kayak and tell her to paddle around a bit that I can then get a quota for her as well?

Seems like a way some people could perhaps take advantage to me.

Yes that what it means.
The people on the boat will of course help you out if you need it. If you drift away from the boat they will come and get you. Same applies to a Kayak. Im sure you could even stretch it to a surfboard if you both went out on it and came back on it.

This rule is actually quite new. Its maybe the 2nd or 3rd season with it. There are plenty of scallops around and if numbers drop Im sure they will look at changing the rules back. I think its quite fair really. It does only apply to scallops though (and dredge oysters I think).

I think the change came about due to the unfairness of the rules governing dredging for scallops vs diving for them.
If you are dredging then you can take 20 per person involved in the prosess. EG 20 for the person on the helm, 20 for the person watching the ropes, 20 for all those the count and measure the scallops.
Of course this made dredging a more appealing method than diving as only the diver could get 20 scallops. But dredging can cause alot of damage to the sea floor. You bring up all sorts if stuff including under size scallops which quite often would not get returned to where they were caught from.

Oh and MAF= Ministry of Agriculture and Forestry

Oh quite right - getting my ministries mixed up haha.