Since you are on the right side of the pond, order yourself on of
Durite 12V 25 Amp Latching Relay - Changeover 5 Pin 0-728-02 | eBay
It's a mechanic latch relay, so the power consumption is momentary and the solenoid valve will always keep its last position. The only problem you might have, as at the initial startup, that valve might be open for few seconds until the vacuum builds up.
I'm talking out of logic only, as I didn't get a chance to test it yet.
As for the vacuum supply line, do not cut anything. Look for existing connection, replace it with a Y and another Y to split to each valve.
I'll try to see if I can find a pic for you.