For drawing unconnected lines you need to find an extra input on the oscilloscope, that enables you modulate (switch on/off) the trace. It may be marked as "Z". Maybe try reading the oscilloscope's manual, if available.
For a rapid, crude test, you can connect directly e.g. P1.0 to X, P1.1 to Y and P1.2 to Z (assuming all of them pulled up to VCC.). Now you have a "1-bit resolution" DAC on both X and Y. Try to write a simple program, setting and clearing the individual pins in a loop, and watch, what you get on the screen.
Now if you are able to display lines, the next move is to place a multibit DAC for X and Y. You might be well off with a 4-bit R2R ladder (the oscilloscope inputs are high impedance so output amplifier might not be needed; but the outputs of the 51 need to be buffered e.g. using 74HC573) with a bit of tweaking to match the etched lines; but the VECTREX circuit is worth looking at.
The lines are redrawn periodically thanks to a loop in the software.

Jan Waclawek