I stand by my 9 for an average gamer. For an above average gamer like myself I could go as low as an 8 but I stand by my 9 because I rate games in the context of how an "average" gamer will experience them.
Have you actually played VR 18 though? There is a huge difference between "seeing" it and actually playing it. I watched a vid on VR18 before I attempted it and thought "Wow, that doesn't look so hard", boy was I wrong! VR 17 - 19 took me about 6 to 8 hours to finish between the three. VR18 took me 4 alone. Some aspects of both 17 and 19 gave me more issues though, but most of those were more about bad luck on my part.
Revengeance difficulty isn't too bad outside of S ranking a few of the bosses and two or three fights where RPG's will ruin them quick and with how random they can be you'll have to restart MANY times till you get a good strategy for handling them while managing the other enemies at the same time.
Its not as difficult as Vaquish but it is more difficult then all the other Metal Gear games outside of MGS2. I voted MGS2 a mid level 9, this I would vote a high 8 to a low 9. I round up, thus the 9.