Can I buy a vehicle and not tell my SO?
Your want a car and won't tell your SO? Why would you do such a thing? He will find out anyways.
Because it's none of his business.
You have a right to feel secretive about some things, but a car is a big purchase. He will find out eventually so do not hesitate to fess up now. He will have felt like you betrayed his trust if you don't say anything beforehand.
I'm a grown man, who wants to buy a car, so I'll do what I want. He has no right to stop me.
I think it is important to talk to your partner before you make a big purchase. Don't you want them to do the same for you?