Permalinks wit WebProxy

Hi! 🙂

For security reason I can only access my /wp-admin/ trough WebProxy/WebVPN.

Because this while I’m posting or uploading media my browser url are different from WordPress default.

Like:
Original url: http://www.mysite.com/
VPN url: http://vpn.provider.com/mysite/

Everything work when I post or add url. But when I change anything in my post, WordPress rewrite all my media url.

Like:
Original media url: http://www.mysite.com/wp-content/uploads/2014/01/kitten.jpg
VPN url: http://vpn.provider.com/mysite/wp-content/uploads/2014/01/kitten.jpg

So when user go to my site, they see only vpn URL (and this don’t work, of course).

Can I force WordPress do not change my url? :/