Calling them dumbc...s and demanding they change their procedures is just asking for a hurting.
Ask VOX, since they're the ones looking to transfer, if it's possible for them to settle the account so the line can get moved, especially since they don't have to pay for it to be installed and will only pay a fee for activation with Vumatel. Don't ask a low-level call center agent either, demand to speak to a manager at least and explain the situation.
If they're unwilling to help, contact Vumatel and discuss it with them in the same relaxed tone of voice. If they're also unable / unwilling to help, just note that you will be invoking Clause 31-33 of their dispute/complain process with ICASA, because since you have no relation to the previous owner, it is unfair of them, especially in terms of the CPA, to hold you accountable by not being willing to transfer the line to the ISP of your choice and that the previous owner should be held liable, who doesn't live there anymore
RE: "just install another line" it all depends on how many fibre pairs are allocated in the area in the first place. Usually the reason why they can't just install another line is because of that restriction, knowing Telkom/Openserve, they probably make sure to have spares available, and thus can easily install multiples at your home. Sometimes they even share the same pair, but that means a degraded experience, which they usually won't go for.
Good luck, keep calm, and don't follow Marcell's advice. Being a 90's kid and shouting/tantrum'ing when you don't get your way doesn't help the situation. They have no obligation to provide you with a service, it's not a human right, so being a name-calling c..t about it, won't help at all.