LARTC - Oct 2005 - MASQed network to exit on different ext IP addres

Hi,
i have a following problem. Looks like this

                                            ROUTER   
--------------DEF.GWa  aaa.aaa.aaa.aaa             
                                                   
|                                DEF.GWb bbb.bbb.bbb.bbb
                                                   |
                         ----------------   -------------------     
                         |                                              
         |                  
              VLAN0015                                ETH0    -----   
xxx.xxx.xxx.xxx
              10.0.0.0/24                            (alias on eth0) --  
  fff.fff.fff.fff.fff

i need that all traffic from subnet 10.0.0.0/24 goes out masqueraded 
with ip address of alias on eth0 (fff.fff.fff.fff)
and have default gw bbb.bbb.bbb.bbb  . There is obsolote kernel 2.2.25 
and iproute ver. iproute2-ss991023 with ipchains version
ipchains 1.3.8, 27-Oct-1998

I tried folowing

/usr/sbin/ip rule add prio 100 dev vlan0015 lookup 15 (here i tried also 
to do prio 100 from 10.0.0.0/24 lookup 15 )
/usr/sbin/ip route add default via bbb.bbb.bbb.bbb table 15   (here i 
tried to add src fff.fff.fff.fff, but no effect)
/usr/sbin/ip route flush cache
ipchains -A fwdmasq -i vlan+ -j MASQ
ipchains -A forward -j fwdmasq -s 10.0.0.0/24

i tried also source nat with the iproute but no way,
/usr/sbin/ip rule add prio 100 from 10.0.0.0/24 nat fff.fff.fff.fff 
lookup 15

The problem is that packets are still leaving router with ip addres 
xxx.xxx.xxx.xxx instead of fff.fff.fff.fff.


thanks a lot
Jorge Sanchez