OK, here's how it goes:
To add 2 or more vectors, make a data chart. In this chart R is the magnitude and 0 (represents theta) is the angle in degrees.
.............R * Sin0 | R * Cos0
.............-----------------------
Vector 1 | |
Vector 2 | |
Vector 3 | |
...
After you fill that in, take the sum of all the sin values and the sum of all the cosine values. The sum of all the sin values is the total Y force (upward/downward) and the sum of all the cos values is the total X force (left/right). So, if you draw a vector of zero degrees with a magnitude of X, and attach it head-to-tail with a vector of ninety degrees with a magnitude of Y, you get a right triangle with the Resultant vector as the hypotenuse. So use Pythagorean theorem and your set!
This is assuming your adding like-vectors. So adding a velocity vector and another velocity vector is OK, but adding velocity with gravity (a force) won't work. Sorry
