I know the X and Y components of vector A (the red line) - I know how to find it's length and angle too, if that helps.
I know angle A (the angle of the green line from 0 degrees, ie. right)
I need to find the vector (blue line) which is perpendicular to the green line, and which goes to the end of vector A.
I then need to find the point 25% of the way along it.
I then need to find the vector from the start of vector A, to that point.
IMPORTANTLY, it needs to work for *any* angle, and *any* vector. It needs to work in all 4 quadrants not just the one shown. I think this is why I couldn't make it work using trig.
Disclaimer: Any sarcasm in my posts will not be mentioned as that would ruin the purpose. It is assumed that the reader is intelligent enough to tell the difference between what is sarcasm and what is not.
Big thanks to everyone who helped - it's much appreciated
I originally started out trying to solve this using trigonometry, as a number of you have suggested.
With hindsight, I can see at least part of where I was going wrong:
Finding the difference between the two lines - I just tried to subtract one from the other, instead of using the proper formula. As an example of why this didn't work, Angle_A = 45, Angle_B = 315. If you subtract one from the other, you get either 270 or -270. The actual difference though, is 90 degrees.
If I'd remembered that then I probably would have been okay, apart from maybe having problems positive/negative angles somewhere down the line.
Anyway, I eventually got it to work using just vectors, so I can't be bothered to try and do it again with trig.