You're about to lose a bunch of money on this building. Trust me. In my mind, the numbers are horrible, there's likely a ton of deferred maintenance, it's been poorly managed, etc. The two vacancies are likely your best tenants.
The reality is that right now there is about $1150/mo in scheduled income, because two units are rented. If the other two units were easy to rent, then they wouldn't be vacant. Don't pay the seller for all the work YOU are going to put into it. You pay for the building as it is now, which is crappy.
Let's say it was completely fixed up, you've done all the work, spent all the money to fix it up, and gotten new tenants (probably 4 new ones, to be honest). Then you'll have a building which is still only bringing in around $2300, which is probably an annual net of $14,000 or so. In which case it's worth $140k at a 10% cap rate, and that's AFTER all the hard work and headache. You don't need to pay the seller for all the hard work you're going to have to do.
You're going to have to put $50k into this project. Offer them $120k and if they don't take it, just walk away. Sadly, if you walk away you won't know how lucky you got. But if you take it, you'll regret it.