You do not need to mind the air pressure too much. Build a room of any size that has sufficient room for a pump (gold amalgam in early stages). The room does not need to be isolated completely and the steam will quickly condense to water, which the room should have plenty of capacity for (1000kg per tile, instead of 1000g per tile for steam).
The pump can then extract the water to any facility/tank you need it at.
So, as you can see, pressure should not be an issue.
But temperature exchange is necessary for this process to happen. The room should not exceed the overheat temperature of the installed pump. Since it is 125°C with gold amalgam and the cool steam vent emits 110°C steam, you should be covered.
In the early stages, this is simply established by not isolating the room too much. But with time it will bleed heat into the nearby areas, requiring cooling of them. However given the nature of the steam vent, this should not be impacting your base infrastructure, as long as you keep your distance from the steam vent when building temperature sensitive stuff such as farms.
It might also be a good idea to keep a medium-size pool of water inside the room to act as a temperature exchange liquid, so that the steam is immediately in contact with a heat exchanging medium.
Outside of the chamber, you might want to place a few wheezeworts (cooling flowers) (or ice machines (if their current balance is not adjusted)) to reduce the impact onto the rest of your base. Cooling the steam directly with wheezeworts is not a practical solution though, since steam contains alot of specific heat energy.
In the later game stages, you can employ more elaborate systems such as cooling pipes or steam generators to condense the steam. But you are far from that stage.
Finally, you might want to clear the atmosphere of the room before enclosing it. So the hydrogen should go somewhere else. Instead of pumping it out, it should be sufficient to simply dig out some more spaces above the vent and wait til the hydrogen sifts out of the spaces you want to enclose. Then you enclose the room and at a later stage you can clean up the remaining oxygen/polluted oxygen that is still in there.
P.S.: When it comes to your electrolizer module, the search term is "SPOM" (Self Powered Oxygen Module)