An alternative to a vacuum cleaner for cleaning carpets

I'm going to assume that the vaccuum was sized to the house, so it's not under-spec, although poor maintenance might mean that it's underperforming.

I'd suspect one of three things -- a blockage, a poorly performing pump, or a leak in the system.

If it's a blockage, unless it's right near the pumps, you'd have some areas with good pressure, and others past the blockage with poor pressure. (and the pump would be straining to keep up). You might be able to map out where a blockage might be by determining which ports have good suction, and which ones don't.

If it's a poorly performing pump, it's difficult to diagnose, but you might try listening for sounds of a straining motor and/or any odd smells. (I'm not so good in this field, so can't really say much more, a bad seal could cause just as bad as a problem, and I don't know if there's a good way to test for that.).

If it's a leak somewhere, the whole system won't be able to come pressure, so you'll have poor pressure everywhere. There might be additional dropoff at the leak if you were to try to map out which ports have worse suction than others, but as the suction is poor to start with, and some dropoff is standard for a pressuried system, it might be difficult to identify.

You might be able to better pinpoint a leak by intentionally clogging up the line, if it's run as a line with multiple ports off of it, rather than them all branching from one point. You'll need a two-way radio, baby monitor, or something similar. Put the transmitting end near the pump, and carry the receiving end with you. Open up the port, and intentionally block the tube on the side heading towards the pump. If there's no leak in that section, you should hear the pump straining.

Luckily, leaks are unlikely (unless perhaps if you've had an earthquake in that area), because I have no clue how you'd go about actually repairing it without tearing out the walls.

Fisrt, DON'T REPLACE THE FLOOR, that would lead to a lot of headaches and is probably un-nessassary. That floor shouldn't absorb oil that quickly if it had been sealed properly. My guess is that the floor either wasn't sealed correctly or it's been so long since it has been that it's all been worn off.

Try getting a wood oil soap (like Murphy's or Old English) these soaps have oils in them that are meant to be absorbed into wood. This won't take out the oil, but it might help to make it blend in. I don't think that stain is comming out, even if you sanded the floor, the stain would have absorbed into the wood too deeply.

It's important to remember that wood is pourous and the natural oils that are in it are evaporated over time and have to be replaced, or else the wood becomes discolored and brittle, which is probably the case here. I wash all my wood floors twice a year with an oil soap to help keep them hydrated.