Allow me to add one more suggestion to what Juan Carlos already suggested.
If I understand your situation correctly, you're trying to draw vectors on a polar plot grid, such that the vectors "move" within the plot.
If that's the case, a common strategy is to draw the animated part of the drawing (in this case, the vector) in 'xor' mode. This imparts on the vector the property of combining with the background in such a way that if you draw the same vector again, in the same location, it restores the original background, thus erasing the vector. This suggests a fairly simple algorithm: before drawing the vector in the new position, draw the old vector one more time in order to erase it. You won't need to draw anything else.
To use the 'xor' raster operation, si
mply set the ATTR_PEN_MODE attribute to VAL_XOR_MODE.
Luis
NI