problem:seperate "internet traffic" and "Lan traffic" in VPN

hi everyone
i have a network with 3 vlan (one of them for server farm) and another for clients.

client can access internet internet by vpn that i define in mikrotik routerboard (1100)(with userman).
each user have a limitation to access internet…but my problem is if user connect vpn and then download or upload any file to servers (local servers in my LAN) , their internet traffic spend rapidly !!!

i don’t want to block access to my local servers when vpn is connect of course.

Please give me a solution to separate internet traffic and LAN traffic when vpn is connect.
thanks
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