From the info page, it looks like Geothermal Pressurizer is intended to be mid-game. However, it has to be placed on a vent, and also needs steam input. Is the intended gameplay process to use a mix of Geothermal Pumps and Pressurizers?

I expected either that the pressurizer would not need to sit on a vent, or if it did, the vent should provide the low-temp steam.

Yes the idea is that the vents is the limiting factor for the geothermal scaling. Indeed a straightforward setup is a few pumps feeding a pressurizer (think of it like putting more steam in your steam to get better steam :D), however you could use an external energy source to provide manufactured steam and use more pressurizer on vents to get more high steam for turbines.

Balancing wise it should still be more interesting in anycase to use the vents with a mix of pumps and pressurizers as it's free steam, the limiting factor again being the vents numbers and the infrastructure investment.

I find it more interesting to have both building in a mixed layout, so one doesn't necessarily superseed the first one when you unlock the second. The pump being also useful for the coal distillery to get oil that also must be placed on vents so you have to chose what to use your limited vents for, steam, supersteam or oil and now with the aquilo tech also heat !

Fantastic! Thank you for the explanation.