Audio- some are stereo already front L&R rear L&R the center and LFE have their own tracks. This is AC3 convention.
Preview screen- You have this set to Preview Auto and anaglyph too. The playback from the timeline is dependent on your hardware. I have three 7200RPM 1Tb hard drives
in Raid 0 stripped for 1 Tb and triple I/O speed. I have 8Gb of fast ram and a quad core. The graphics card is nvidia 8800 GT and the CPU is a Q9300. You can compare what's different in your system as I can play 5.1 audio one track video at full HD from the timeline. I use preview auto so it will keep up for previews when I'm working with up to 4 tracks of HD video. ( I assume that would be equal to 2 tracks of 3D MVC video. which so far has performed but does hesitate with a third track in play even with graphics. As I said it's all about making the hardware strong on all fronts. This aint no little SD edit system anymore.
Outside of doing 4K ( which Vegas can do these days) were doing the most demanding video editing in the industry with full HD and 3D.
If your preview was set to follow your project properties then the clips should have been in anaglyph. Right click on your preview window and check it out to be sure. Preview device settings is a second monitor, check out Video preferences and set it to use project settings.
When you put your clips on the timeline did you see the video go to two video tracks one above the other or a single track? If two tracks then they came in not paired for 3D. That tells me your project properties was not initially set right. If one track then you did it right. You should not see each left and right video if they are paired. If you do then the only thing you will see in the output window is the top track in 2D as it will mask the video under it. Always make sure you set your properties for stereo before you bring in any 3D clips. The timeline should be blank with no tracks and with stereo on and anaglyph, the tracks generated as you drag your first clip to the timeline region. This will create one video track and 4 audio tracks from the Sony MVC file. If you need more video tracks for editing other clips, you can add them by right clicking on a empty region of the existing single track and get a menu to add video or audio tracks. You can also re - pair the unpaired L&R clips by selecting them and then right clicking and choosing the 3D stereo effect too just like you would if bringing in twin camera files.