I have never seen a package do anything like this, and frankly it is unacceptable and totally unnecessary. We don't configure daemons for you so
1) Why the hell are we even running this --testkernel thing? It totally blocks pacman for **60 seconds**!
2) Why are you showing me all this garbage on install? Of course I have to configure the package.

(1/1) installing vnstat [######################] 100%
==> Testing the kernel ...
This test will take about 60 seconds.
[==============================] done.

Detected boot time variation during test: 0
Maximum boot time variation set in config: 15

The current kernel doesn't seem to suffer from boot time variation problems.
Everything is ok.

==> Every interface you want to monitor has to be introduced to vnStat!
==> eg. vnstat -u -i eth0
==> An example of a crontab to update vnStats database every 5 minutes
==> has been put in /usr/share/doc/vnstat/examples/vnstat.cron
==> Additionally, vnstat includes a daemon which can be added to your
==> rc.conf to update the database.
==> DO NOT HAVE BOTH THE CRON AND RC SCRIPTS ACTIVE AT THE SAME TIME.
==>
==> Webbased statistics can be generated using:
==> /usr/share/doc/vnstat/examples/vnstat.cgi