Check system VPN statistics? Is there any way to check VPN stats for stock PPTP VPN applet? Maybe some stat-file or GUI-way? If not, can we utilize Tasker for that?

I particularly interested in uptime indicator, i.e. was my VPN disconnected during last 24h or not?

I saw this question and this app, mentioned in the question, but it was last updated in 2015 and seems to be not working on Marshmallow, which I am on (LG G3).
Since you mentioned Tasker, try setting up an appropriate task for the profile State &rarr; Net &rarr; VPN Connected. No clue if it works on Marshmallow though.