There are currently 3 vector operation to choose from in gcalcrpn, dot products, cross products, and magnitude (abs). To access these functions press the vector button from the main display and then choose perform vector operation. Remember that the dot and cross product operations are binary operations so they require 2 vectors as operands.
You can also perform operations with vectors and real numbers/matrices on the stack. You can multiply a matrix by a vector as long as the sizes correspond make the multiplication legitimate. The output of this operation is a vector. You can also pre or post multiply a vector by a scalar. this is done by entering a vector and real number on the stack and pressing the multiplication button.