I'm setting up a L2TP/ipsec server from my mikrotik to be able to remotely login from my phone.

Can i choose an ip range for the vpn to be in the same subnet as the lan connected to my router?

I would like to connect to a camera on my lan from the remote vpn device.

Do i need to use proxy-arp ?

Yes you can use proxy ARP for that but it's not true layer 2 adjacency. If your camera solution requires that you'll want to look at BCP or PPP based bridging to see if that works on your phone. Otherwise find a camera system that works under normal IP routing scenarios. Proxy ARP is a crutch, has security implications and generally is a bad practice for you to rely on it.