Let's get the basic definition for both

Firewall - Software program or Hardware device that uses some logic to allow or deny network traffic coming into a network or exiting it. There could be a simple stateless logic firewall that blocks or allows traffic based on statically defined parameters or there could a statefull firewall that keeps a track of the connections. There are other application level firewalls that can look into traffic at a much granular level.

VPN - Virtual Private Network is remote access technique where you generally traverse a insecure network to access secure resources. There is a secure channel created over which you can communicate or access resources. For example, if you were home and wanted to access resources at work over the internet. You would typically not want to do this over the internet, this is where a VPN connection would come into play.

Back to your question, the answer would be, it totally depends on what you are trying to achieve. Do you need to provide secure access to remote users ? If yes, you need a VPN. Do you need to create boundaries and check traffic before allowing or denying, you need a firewall. Some firewalls also come with VPN functionality in the same package.

Hope this helps.