Just communicate where it's convenient for him. If he doesn't want to change his habits after 15 years of friendship, there's no point trying to convince him - people rarely change in these things, and pushing will only damage your relationship. VK works fine, messages come through, you can see and hear each other. That's not a problem, that's just how he prefers to communicate.
It's different if you need something urgent and he takes a long time to respond on social media. Then you could work out an exception: like, "if something's critical, I'll call." But if your regular messages go fine, then really, there's no reason to obsess over it. Everyone has their quirks - some people don't read long texts, some are afraid of video calls, some just prefer old-school SMS.
The main thing is that you're both okay with the communication channel. If it only works for him and it's constantly inconvenient for you - then you can be honest about what you need. But if you've both found VK to be an acceptable way to stay in touch, then it's just the normal for your friendship, nothing weird about it.