capturing captcha in vuGen script for login transaction to be run in SiteScope web screipt monitor

Hi All, We are going to monitor some URL login transactions using VuGen script and SiteScope web script monitor. But the issue is that for every login there is a numerical string(captcha) generated by application and we think there could be a issue when we run script in SiteScope. While recoring script we will give one value which will be generated on login screen, and same will be recorded in script as well, but then how will SiteScope run the same login script to check response time, as there will be new string generated by application which needs to be entered to login to application. Please elp on this matter. Regards, Hanumant