For a long time, net/vnstat did work fine. But since some days (weeks?), it started to fail to log all traffic occasionally, and it gets the bandwidth of my ppp(8)-tun(4) interface wrong everytime. When I notice that it fails to log the traffic, I have to restart it manually, until it starts to log the traffic. I guess this behaviour started with the last update. I'm using packages on a stock FreeBSD 12.2-RELEASE-p4, amd64 laptop with internet from WWAN (mobile phone radio network) with a built-in Ericsson HSDPA+ modem.
Please note the bandwidth applied to the tun0 interface (excerpt from /var/log/messages.log):
Mar 18 16:25:06 t450s vnstatd: Monitoring (5): wlan0 (300 Mbit) ue0 (2500 Mbit) tun0 (115200 Mbit) ipfw0 (2500 Mbit) em0 (1000 Mbit)
but in vnstat's config I set (/usr/local/etc/vnstat.conf):
Note that 115200 baud is the default speed of the virtual serial line to the modem; that has never hindered the modem to supply bandwidth of up to 18 Mbit/s (usually ~1-12 Mbit/s, my ISP's max. is 20 Mbit/s).