The ‘open and free’ internet is standards based, so you cannot force port 587. If what you intend could be done then why would Vultr block just port 25? They would go on to block 587 as well, no?
They want to take your money but they don’t want you to send out unlimited email from their IP address.
However, there is a way forward for you. See Free smarthost / mail relay to work around port 25 block by VPS host