It is never rude to make an offer, period. I think it's nice that you worry about appearing rude, but I always tell my sellers, an offer is an offer. It means somebody cared enough to take the time to consider your property. Hopefully, you can meet somewhere in the middle and both parties can feel the end result is a win-win.
As to your specific question, I don't know what's going on in Vancouver, so hopefully somebody will chime in here, but I can tell you that a fixed percentage off ask price cannot effectively be applied across the board. Without knowing if the property in question if priced right to begin with, it is impossible to say whether or not 10-15% would be lowball, just right, or heck - even too much!
Put your head together with an area Realtor who can present you with solid "sold" comps, and formulate an offer based on what is happening right now in the neighborhood.
Good luck to you!