It is a mistake to think of it as a vacuum casting machine, it works
by removing the air in the mould and using the weight of the air
above you to aid gravity and push the metal into the mould.
You do not have as much air above you so much less push, you cannot
ge a pump that gives you more vacuum, only vacuum for a longer time
when casting or faster evacuation when degassing investment.
If you are getting surface bubbles, adding slightly more water or
increasing the water temperature will help, you will probably have
to adjust ,ie shorten the total work time as a result.
If you are not getting complete filling the extra water in the mix
may make the plaster slightly more porous, not sure about that, or
you may have to go the centrfuge route
regards Tim Blades.