What steps have you taken to determine it is an issue with the timing?
The main function of the vacuum advance is to advance the timing while you are cruising down the road while not under load (think going down the freeway on the flat). This is to boost fuel economy. When you are going up a hill, you have your foot in the throttle and almost no vacuum to the advance on the dizzy so, that should not be affecting your hill climbing abilities.
You also have initial timing and the mechanical advance in the dizzy itself.
