Can someone spy on my phone with VPN?

I've been hearing a lot about VPNs and their ability to keep my online activities private, but I'm still concerned about security. Can someone still spy on my phone if I'm using a VPN? I understand that VPNs encrypt my data, but are there any vulnerabilities that a hacker could exploit to gain access to my personal information? Additionally, what steps can I take to ensure that my phone is as secure as possible while using a VPN?