If you were my client, I would be very leery of buying a property that cannot be delivered vacant because then you would assume the responsibility for eviction. Too many horror stories about people closing the transaction only to be smacked by an uncooperative resident, whether they're legal or not.
If the bank can't or won't evict the "owner", that raises huge red flags.....
If you buy the property and title is transferred to you before the current owner is evicted then you will have to do the eviction when you close escrow. You can pay a lawyer to do that for you, it is relatively inexpensive. The eviction will take from 30 to 90 days.
The bank probably does not want to evict the owner because it fears vandalism could take place once the owner is evicted and the property becomes vacant. This, of course, is only my guess.
Suggest you read the preliminary title report to see if there are any items which may catch you by surprise. If you are not confident about what you understand with the Preliminary Title Report, give me a call and I can help you (408-316-0793)...or email (firstname.lastname@example.org)
Quickie: Rent control & "just cause" ordinances are not 1 in the same; people erroneously conflate them all the time. They are complimentary & at times intersect, but cover 2 very different dynamics.... and may be wholly irrelevant to this situation. But... it depends... so your best bet is to work with an agent experienced in these exact matters.
So reach out to me & we'll see if we can work together....
Ron's suggestion below to offer cash for vacating without a fight should seriously be considered, as should an attempt to have the lender evict them before close of escrow anyway...or negotiate a better price to compensate you for the legal expense and hassle.
It is harder to get someone out than to get someone in.
The Homeowner may be getting Cash-for-Keys after moving, which would give them an incentive to move quickly without trashing the house.
You might be proactive in this by offering them your own incentive: Possibly $1,000 with a Contract that says that they will move by such-and-such a date, without trashing or removing any fixtures.
You have a Realtor, what does he/she say?