VIP vlan and Server vlan on single trunk between F5 and L3 Switch

Is there any reason to have vlan 10 configured on the bigip?

In this case, serverside traffic exits on vlan10 and response from server arrives on vlan20, as the VIP address is the source-ip of server-side traffic.

Since, the server's default gateway is the L3 device and server is connected to it, you can use just vlan20 between the L3 device and bigip and remove vlan10 on bigip. To connect to the server in vlan 10, the bigip will send the traffic to it's default gateway. This way serverside request and response will exit and arrive on the same vlan.

You can also use an irule like this to use the vip address as the snat ip for serverside connections.

when CLIENT_ACCEPTED {
  snat [IP::local_addr]
     }