It depends where you live and your country’s international bandwidth to the US. If you can get 10 Mbps locally, you should be able to get enough for HD (unless you live in the middle east or australia cause their connections to the US isn’t very good).
You need about 5 Mbps minimum download speed to get full HD (4K).
If you have 10 Mbps download speed and use a VPN, your speed will be reduced. How much? That depends on the VPN service and your route to the service. If the VPN server has too many users or does not have enough bandwidth, the users will see slow speeds. If the route from the user to the VPN server is poor, the users will see slow speeds.
The 25 Mbps requirement/suggestion by Netflix is to ensure that the video playback reaches 2160p almost immediately after launching a video title. However, in testing, I have found that between 5-10 Mbps will get you 2160p within a minute or two of read-ahead buffering.