Suitable amplifier for relay controlling solenoid valve
What is my easiest, cheapest option here to still use the vacuum gauge relay to control the solenoid?

The simplest solution might be to find a 2nd relay that can handle the solenoid voltage and current and, activate it with a 12 (or 24) volt supply using the current 30 volt contact. Don't forget to put a flyback diode across the relay coil and the solenoid though.
Alternatively, you could choose an N-channel MOSFET rated for about 60 volts or higher and capable of handling 3 amps or more. You'll still need a flyback diode across the solenoid and, you'll still need an auxiliary supply to drive the gate but this time it should be limited to 12 volts in case you damage the MOSFET gate-source region: -