@Sindy, @mkx, thank you for your valuable input. I tried different ways to configure the AP. But still cannot get the WiFi working. Everything else works well.
Therefore, I want to post the configuration for the trial set up. Have been struggling for a week but no success. 
Router: RB951G ( ether1 as WAN, and ether2 as trunk to Switch)
Switch: RB951-2n ( ether1 as trunk to Router, and ether2 to cAP)
AP: wAP (local forwarding mode and controlled by CAPsMAN on Router with 2 SSIDs)
Configuration: according to pcunite’s guideline and wiki: CAPsMAN with Vlan
Issues: Ports of ether 3-5 on router, ether 3-5 on switch works perfect. WiFi doesn’t work, either cannot connect to the SSID or extremely slow.
3/26: I tried in the past few days and herebelow are my observations on the WiFi :
- CAPsMAN with local forwarding mode: Cannot use Vlan filtering on CAP, turn on vlan filtering feature will lose internet connection
- CAPsMAN with manager forwarding mode: very unstable with vlan filtering on, sometimes you can connect to the SSID, sometimes, iPhone is unable to connect to the SSID.
- Most weird thing I noticed, if SSID1 is assigned with vlan10 and SSID2 with vlan20, the SSID1 is always faster and more stable than SSID2. Seems there is a priority. I know there must be something wrong in my configuration but I cannot figure out.