Should I use a vpn on my MacBook Air?

That's a matter of personal preference. Macs have all the security they need built in. If you need added security for some reason you might want to, But since nobody knows any details of what you're concerned about, it's all up to you to decide.

You need a VPN to create a secure point-to-point tunnel to your workplace or other network. Talk to your network admin to discuss their recommendations.

You do not otherwise need a VPN. A public VPN routes all your traffic to the VPN server. Why would you want to route your private or finacial information to a third party to do with as they please?

OK, it's still up to you. There's no absolute guarantee a VPN will protect you from anything. The best defense is strong passwords you can remember, be cautious in everything and check often to determine if mischief has been done.

Any plan to protect, should also have a "disaster recovery plan". That will mean different things to different people, to us it means if worst comes to worst we have a way to recover all data in multiple ways and are able to withstand a financial systems failure.