You should add env=!dontlog
at the end of your CustomLog line.
Example (I also noticed the backslashes were not necessary):
<VirtualHost *:80>
...
SetEnvIf Remote_Addr "192.168.1.1" dontlog
SetEnvIf Remote_Addr "192.168.1.5" dontlog
SetEnvIf Remote_Addr "192.168.1.10" dontlog
...
CustomLog /var/log/apache2/mywebsite.access.log combined env=!dontlog
...
</VirtualHost>