It is possible (likely, actually) that you rendered to a different frame rate. When playing in a media player, any frame rate should play OK. However, if you put that video into Vegas, and your Project Properties are not the same as the source video frame rate, the resulting playback will be slow and jerky because Vegas is resampling the video in order to make it play at a different frame rate. This is extremely computationally intensive, and Vegas can't keep up.

Solution #1: Set the Vegas Preview Quality (button at the top of the preview window) to "Preview" rather than "Best." This is quick, and will help, but is not the best solution.

Solution #2: The better solution is to go to the Project Properties and select the "Match" icon in the upper right corner. Then, navigate to the folder that contains your video and click on that video. Vegas will then match the project properties to your video and will no longer have to labor to make that video play at some other frame rate.

Finally, when you do your render in Vegas, always make sure that the template you choose has the same frame rate as your source material, unless you really and truly want to change to a different frame rate, something that I would generally advise not to do.

A good tool to have for situations like this is the Mediainfo tool (use Google, it's a free utility). It provides information about bit rate and, sometimes, field order. It can quickly uncover problems like this. If you already have the tool, take your rendered file and drop it on Mediainfo. Then do the same thing with your source video. The bitrates will be different, and also possibly the horizontal and vertical pixels, but the frame rate should be identical.

Agree, make sure the frame rates match. Also, not sure if wmv supports interlaced, but if your original is interlaced, try swapping the field order on the original or the render setting and see if that does anything.

Since we shoot and hand off AVCHD to Bluray, folks incorrectly assume it I a delivery format. It is not, and playback can be a crapshoot

If your bluray plays, everything is fine. For PC playback render an mp4.
And remember that Vegas preview is not a transport stream player.

Thanks for the replies guys. In response:

PeterDuke

The problem is both viewing the original file in Vegas and the AVC that Vegas outputs. It has nothing to do with preview quality. Playback of orignal and output is still jerky even set to Draft (Auto). Bitrate of original footage is only 8500 kbps and I get smooth playback on footage with bitrates in the 30 Mbps range.

johnmeyer

As stated above playing around with preview quality has no effect. Playback in Vegas is jerky no matter what preview quality I try and my machine has plenty of ooomph to play back high bitrate material in Vegas.

I have Vegas set to always base project video settings on first-added video. Notwithstanding that I did try your second suggestion and went through the process of manually matching properties, but no cigar I'm afraid.

The frame rate of the original material is 29.97. All the Bluray render profiles seem to be 24p, 50i, or 60i. I don't see an option for 29.97 and have been using 60i which I understood (perhaps wrongly) was the correct profile for 29.97 material for Bluray. I've used 60i for a large range of 29.97 material in the past without encountering this problem.

Finally,

To me it seems like Vegas is having a problem reading (if that's the right word) the original material. The problem with the output is that it is faithfully rendering what it reads from the original material as jerky video. I'm wondering if the original material might be encoded with a variant of the wmv codec that Vegas doesn't recognise causing it to default to a less compatible codec. Clutching at straws here.

Thanks again guys, and I welcome all comments and suggestions.

Any chance you could upload a few seconds of video that plays slowly on your timeline? That would help us determine if the problem is the video itself, Vegas, or your computer.