VRT evasion

I think its a fiar point, there seem to be a lot of people driving NI or UK registered cars and it's a form of tax evasion after all if they're not following the rules

One I've seen clearly purchased the car in NI new and now drives it here, how's it that supposed others who pay VRT etc. feel?

I think you might just make it worse. My friend was having a similar problem with their neighbour whom it would seem is bringing cars down from the north and using car park spaces in the apt complex to hold them before he sells them. Anyway, bfs boyfriend threatened him with customs and excise after a run-in one day and the guy got really really aggressive and they ended up going to the Guards. I would tackle her anti-social behaviour rather than making threats which I think could make her even more agressive. I feel for you - this is an awful situation to be in.