"Does anyone know if there is a way to avoid this strobing? "

Levels...
this strobing is usually caused by interlaced flickering. and is prominiant on slow motion with strexthed levels (RGB) luminanc is also a cause of this phenomena.
I found by decreasing levels, this strobe issue is avoided..

It predominately happens on progressive footage which has been converted from 50/60i source material

"any solution for this in Sony Vegas?"
For teh strobe, same thing.. jsut run your levels filter and make sure your colours are broadcast safe..

As for the slowmo, there are many ways to get this kind of slowmotion... Vegas is pretty intense with this stuff, but what DONT you know about vegas slowmotion?

Id recomend you put a post up in the vegas forum, or search a reply to a thread i made (about 2 yrs ago or somethign) where i went through the notions of explaining a variety of different slow mo techniques using vegas..

at the 12 second mark, if u watch the lady in red jacket, youll notice a jaggy stagger/flicker.. is this in the master render? Or is this a nuance of the web compression?
Was this done using native HDV or did u use intraframe intermediate?
Mac i know, but im not sure how QT conforms HDV for edit..

Also, try running the levels by about 1 stops lower than what you have, and mayb run a desaturation filter 9or maybe even a BW filter at 15%) to calm down the warm tones

as for the HVX, i really wouldnt knock the cams cabilities.. the DVCProHD format is incredibly robust and overcranking the camera saves a copious amount of steps. When you consider the steps you took to get these result,s vs the incam full resoluion slowmo abilities... well.. u can see how different they are..

In addition, your results on teh HXV are viewable immedaitely upon completing the shot.
Its a totally different beast and the slowmotion workflow is very different to a Post Prod slow mo workflow.. . IMO, the 2 really shoudlnt be compared as not only are they different formats using different colourspace, but also the inter & intraframe differences will determine how well the final result will look compared to the other....
Theres a lil more to it than what ive written, but if you want to slow down interlaced footage, i wrote up a huge response (in teh wedding forum from memory) which details how reference frames are used to draw in new interpolated frames.. it also goes on about how to offer the NLE sufficient reference frames/fields from using 60/50i sourced material

Wouldnt call myself an expert..

supersampling works like audio oversampling.. it takes multiple scans of teh source material and then process as required after running these multiple passes.. in a basic sense, u can look at supersamplin in vegas as a hardcore variation of multple pass encoding.. in realtime..

The supersampler works a treat for slowmotion, as well as for compositing and track motion. Also supersampling works miracles when interpolating for various frame rates (PAL to NTSC is prolly the most common) as well as "cranked" framerates...

By overscanning the source material, a larger reference database is created so as the interpolation of cranked footage takes place, it has more information to work with to draw the frame in question. Also Vegas uses a variety of different interpolate/frame blending methods, in addition to render algorythms which not many other apps offer..