with local aaa authentication
go to the user atributes
like username vpnuser attributes
vpn-framed-ip-address 192.168.50.1 255.255.255.255
this will give that ip to that user
if u are useing cisco ACS
under the user setting
go to :
Assign static IP address-If a specific IP address should be used for this user, click this option and type the IP address in the text box. The IP address assignment in User Setup overrides the IP address assignment in Group Setup
and the following link give step-by step intstruction to configure cisco ACS AAA
http://www.cisco.com/en/US/products/sw/secursw/ps2086/products_user_guide_chapter09186a008007e6a6.html
good luck
please, if helpful Rate