From my understanding, V6F is the previous name for the factory , which changed a couple of years back to VSF. The first decent iteration of the Spectre was made by V6F and progressed to version 2.5 around 2016/2017. In 2018 V6F, now VSF, upgraded the watch with a clone movement and better finishing overall. It was also at this time that MKS had a decent rep of the Spectre. The MKS Spectre could be had for a better price, but had flaws ("Jewets" instead of Jewels printed on the movement for example), this allowed VSF to overtake it as the more popular choice.
Since 2018, the VSF Spectre may have had a silent upgrade or two; such as, a more grainy dial, but as far as I'm aware, the watch hasn't changed it's version number. The watch at this time is labelled as the VSF Spectre. If you see a Spectre labelled 2.5, I'd be inclined to think it was the older version of the watch, made when the factory's name was V6F. This is from my understanding, if anyone knows differently please chime in.