Contrary to the other person’s answer, the condenser will NOT collapse. The MOST vacuum you can obtain is only a 15psi differential across the walls of the condenser. Remember, the condenser only has to keep out atmospheric pressure. It is NOT like a boiler which is designed to CONTAIN pressure and therefore must be quite a bit stronger. The design vacuum is the vacuum that it works best for HEAT TRANSFER and has nothing to do with the pressure differential. In any steam system, the vacuum is initially established by eductors drawing out the air. The vacuum is then maintained by the steam exhaust from the turbine or engine condensing. All that happens if you change the amount of vacuum in the system is to change the temperature where steam condenses, which will reduce the overall efficiency of the system.

Increase the Vacuum means that you are lowering the absolute pressure. A perfect vacuum of 30 inches of mercury (inHg) equals zero PSIA (pounds per square inch absolute)
You cannot create more vacuum than this. Typical unfired pressure vessels are rated to 0PSIA as it is possible to easily establish a High vacuum close to thirty inches of mercury from the collapse of condensing steam (after non condensable gasses have been purged).

So to answer your question: basically nothing happens other than establishing a high vacuum.

However, depending upon the use of the condenser there will be other impacts to system operation. A condenser attached to a turbine exhaust will become more efficient with a lower exhaust pressure in the condenser. Also hotwell condensate temps will be lower with a lower PSIA in the condenser.