Match your project video property settings to your source footage. When you do this, Vegas Pro software has fewer calculations and conversions to perform and can thus play each frame of the timeline more efficiently.
In order to be totally sure that your project properties match the properties of your footage, sample one of your files and let Vegas Pro software specify the proper settings for you. To do this, click the Match Media Settings button in the Project Properties window. This opens the Match Media Settings dialog box. In that box, navigate to the location into which you’ve stored your video files, select one of the files from that location, and click Open. Vegas Pro software adjusts the settings in the Project Properties Video tab so that they exactly match the file that you sampled.