I said I was pretty sure, but added the caveat that I wold need to know when your 75% Viper mast was made.
In the 90's, I sailed all top of the line North sails and masts. The North "version" of a "constant curve mast was a bit soft in the top. Almost a flex top. If your North Viper mast is significantly newer, it may truly be a 460 75% North Viper that meets the IMCS 24-26 spec.
When I switched to Sailworks sails, the North masts didn't work very well as they were not truly
I do feel that even a beginner to intemediate sailing skill level, you will impede your progress and reduce your overall fun factor with a mast that not very similar to the mast the sail was designed on.
What are the performance factors that even beginners will notice:
1/ If the mast is too stiff for the sail (this happens when you use a 460 mast in a sail designed for a 430 mast and you just extend the head strap) you end up with a sail that doesn't rotate well, does not twist off in gusts the way it was designed, and you end up pulling alot of the power and stability designed into the sail out of it when you downhaul it as the middle flattens out due to excessive downhaul.
So the sail is gutless, does not rotate well, does not twist off well, and seems pretty unbalanced and "twitchy".
2/ Conversely, if you use a mast that is too soft in bend ratio, the top of the sail twist off way too soon, the middle of the sail battens never pull back as they were designed to do, so the sail does not rotate well at all. Also you will not get the depth down low or much stability.
Can you sail with an incompatible mast? Absolutely, but if you ever get to rig that sail or have someone who reall knows the sail rig it on the correct "best" mast, the whole rig will come alive, every thing will rotate smoothly and with very little effort. The sail will feel very nicely balanced, and have a far greater range of use.
3/ No, there are probably no real "safety concerns beyond the minor potential for you to stress the mast in some manner that causes it to break.
4/ Speed will be better (because the foil will be correctly proportioned and positioned in the sail,plus with good balance you will feel more comfortable and confident that the sail is going to support you so you "go for it" because a correctly tuned sail is an aboslute joy to sail.
5/ Maneuverabilty....if you mean the way the sail handles in transitions (tacks/jibes/jumps, yes a well tuned sail on a truly compatible mast does all types of maneuvers effortlessly.
6/ Planing capability.... Unless the mast is compatable, you won't get the depth of foil down low that will get you planing quickly. Also, if you are "tuning around" a mast compatabiity issue, you won't be able to tune the sail really as full as it was designed to go (less downhaul and less outhaul usuallly) to get the absolute best low end power from it .
Hope this helps,