Just talk to Varel about her Joining and she will become a Grey Warden. This can be done at any time after she joins your party—although for story purposes it does not make sense until the Silverite Mines are completed and she asks to become a Warden.
Also, if the Wending Wood was the last place you visited before the main battle that the Seneschal prompts, she cannot become a Grey Warden unless you travel to Vigil's Keep directly after Velanna joins the party AND another companion's joining quest is open (PC 1.04).
- Velanna becomes a Grey Warden.
- Velanna gains +1 Talent point. The same occurs for all successful Joining rituals.
- Justice disapproves (-10) of Velanna escaping justice by joining the Grey Wardens.
- Oghren disapproves (-3) when you first recruit Velanna (before you go into the Silverite Mines).
- Anders approves (+2) when you allow Velanna to become a Grey Warden.
- Sigrun approves (+3) when you allow Velanna to become a Grey Warden.
NOTE: For PC V1.5, none of the approval changes stated above would happen. Tried reloading an old save, and still nothing changes (party/individual member approval rating below maximum).