How to prevent play store from detecting VPN?
One of the most successful methods I use (which you can also use in this case) to bypass VPN detection is using an anonymous or elite proxy.
To implement the proxy on a per-app basis you can use ProxyDroid or any similar application.
After connecting the VPN simply grab an elite/anonymous proxy from paid or free sites like this one: hidemy.name.
Open ProxyDroid and input the elite proxy and filter to use with Google Play (or other apps if desired).
Connect and download.