you absolutely cannot be naming your company after a domain name you do not own. Terrible choice

Maybe vvx.dev? Naming your service a domain you don't own is going to be confusing and possibly result in legal headaches. Not worth it

Or just vxdev or vx-dev or VxDev or something that doesn't look like a domain name.

I mean it's not like you can name your company Microsoftcom so I'm not sure these are any better. If someone owns vx.dev and they bought it before this thing was published they don't have to try very hard to make a case for infringement.

It's not like the letters "VX" are now permanently claimed and unavailable for any software-related product.

This might depend on things like whether the name of the domain owner is "Vx Dev", or "VX Systems Inc" and they happen to have a cute .dev TLD.

If your product takes off, it for sure won't get cheaper

sounds like you need to rebrand immediately