Create a new project. Set the audio to 5.1. Create 6 audio tracks on your timeline. in the surround pan box on each track, move the orange diamond to the speaker that corresponds with the surround channel you want to use that track for. Make sure you then mute all other speakers that are not being used for that track by clicking on each one. When muted the unused speakers will be a lighter grey. Right click on the surround box on each track and set the pan type to “Balance 0db” for the first 5 tracks. on the 6th track, right click on the surround box and set that to “LFE only” . Import your mono wavs and place them on the corresponding track. Now you can export them to wav64 using the “Sony wav64 - 48,000hz 24bit 5.1 PCM” preset. Don’t have “enable multichannel mapping” ticked. then export you file. When you want to import it back into vegas , import the file then drag it onto your timeline to create new tracks ( a blank area below your existing audio tracks). Don’t drag it onto existing tracks as this can mess up the channel mapping. It will create 4 new tracks: Stereo F&R, Mono Centre, Stereo SL&SR and LFE.