I'm interested in investing in vertcoin, but I have one concern: Is there anything stopping other scrypt coins from implementing Adaptive N-Factor, thus diminishing the appeal of vertcoin?
Other coins can hardfork to variable-N but the disruption, inconvenience and market uncertainty that comes with a hardfork is a big disincentive. It's why I personally pushed so hard for our fork to happen _early_, before we had issues with difficulty strandings, and before VTC got so big (ie. even now, and certainly in future) that doing so would be more disruptive.
So no, there's nothing preventing them from doing it, but there are considerable reasons not to do so as well.
I think the core issue there though is, why would VTC clones diminish the appeal of VTC? BTC hasn't been dimished by PPC/NMC/FTC etc, and LTC hasn't been diminished by DOGE/MEC/WDC etc. In fact, if anything, they have been strengthened by contrast - the alts are seen as the speculative, small-cap gambles, or what people mine to sell, wheras BTC and LTC have become the reserve currencies that people are comfortable holding in.
VTC is unusual, in that enough people realised early that holding in it would be the right decision. I think that's because of the dev support behind it, because it's not premined, and because of the lack of hype - it doesn't need it.