Hi,
thanks - doing my best...
I must say i have no experience with twinser freestyle boards at all... i heard they are sticking to the water more than single fin boards, especially the RRD seems to be more sticky, which would explain why you feel that you have difficulties to pop in underpowered conditions.
Anyway, for the mast foot, i think its a bit different from board to board... the important thing is that you never move the mastfoot more than 0,5-1cm for testing... it makes a big difference where it is positioned!
My experience is, that modern freestyle boards normally have one position for the mastfoot where it is best for freestyle tricks. there, the board rotates best and you have the best pop somehow... I found many times this position is a bit behind the center, but this is somehow totally up to the shapers of the board and where they put the mast track.
Moving it more forward maybe just helps you to pop better cause you can plane better. I am not sure cause i don't know that board exactly, but it might need the mastfoot to be a bit further forward to be more flat on the water. So you can plane easier and quicker. If you put the mast foot too far back, the board will turn better for carves and also in the air when doing air jibes, but you might have difficulties to get planing, cause the board's not so flat on the water. Also landing can be more difficult like this.
All in all, you are doing well if you can feel the difference - many people ignore the feeling of board and sail when learning moves and think they wouldn't feel it anyway. The thing is that you feel it indirectly,... as you discovered - it makes you sometimes land a move or not...
The Vulcan - if you don't slide backwards at all, well, it seems the board might be too small for you, you are not pushing against it at all after landing, or the wind might be super light

- its just normal that you won't slide that much when the wind is very light...
I haven't seen your vulcans, but let somebody film you and check when you grab to the other side of the boom - if you are landing and at the same time have your new mast hand on the boom already, its fine... if you are landing and have your new mast hand somewhere stretched out upwind to balance and then grab the boom or mast, you'll need to improve your grab technique... this is the biggest problem people have with vulcans... grab around to the new side of the boom right after take off, in the air!!
For spocks just go faster and go downwind a bit more. Don't jump too high and try to land in a flat area on the water... when sliding backwards, extend your back leg and push the sail upwind with the mast hand - keep the sail hand relaxed, don't extend it fully and don't twist your body, keep the line of back foot and mast arm strong...
You are doing great!! you should slowly start trying flakas...

Willy Skipper too,...
great you like the book! Get the DVD too if you go to the next moves, there you'll see and learn quicker how a move should feel and look...
Let us know what are your conclusions on vulcan and spock and the mast foot position...
all the best
Rossi