Train Vienna --> Prague

Sounds about right. It was an old generation slow train when we took that route 20 months ago,

Yes. Trains cannot go any faster than the quality of tracks they are running on. French TGV's run on specially built tracks that allow them to go 200 mph safely. As soon a TGV goes to normal tracks, their speed drops to the 60-70 mph range. The tracks from Vienna to Prague are normal tracks.

Tracks from Vienna to Prague are not normal tracks. They are so called corridor where speed is up to 160 km/h = 100 miles per hour.
However not TGV speed.

Railjet trains on the Vienna - Prague corridor are relatively new, but they are the new normal. Direct trains run about every two hours, daily. Do yourself a favor and buy direct tickets, rather than one that requires a change along the way. Will take 3 hours 59 minutes. You can depart from Wien Meidling or Wien Hauptbahnhof. The cities are 205 miles apart, so a 4 hour ride makes sense, yes?

The route taken, Breclav, Brno, Ceska Trebova, Pardubice, makes it more like 250 miles. Since it stops at each of those towns and sits for 2 to 3 minutes in the station, 4 hours is pretty good.

It's much easier and nicer to use a shuttle service. Door to door service and reasonable.

We used CK Shuttle and we were pleased.

CK Shuttle usually makes sense if you want to go to/from Cesky Krumlov, but between Vienna and Prague it costs $80 per person and is slower than the train (driving via Cesky Krumlov).

As Sam says the distance between Vienna and Prague is indeed 250 miles. So doing it in little less than 4 hours is not that bad. The stop between Brno and Pardubice is Ceska Trebova.
I would not use shuttle between Vienna and Prague. The only advantage is door to door. Justified for very heavy luggage and walking impairment. Disadvantages: more expensive, not as comfortable as train, not as fast but can compensate for lower speed being door to door. But if you get stuck in traffic jam on the freeway between Brno and Prague then you loose big comparing to train. Yellow bus makes Brno to Prague little over two hours. Once it happened to me it took five hours. Since then I go always by train.

Took the Vienna to Prague Railjet in early September of last year. Very nice and, yes, a 4 hour trip.

I doubt there is a night train between these locations as the time period is too short, right?

Surprisingly, there is, but you arrive at 4 am.

Seat61 says price for 2nd class is 19 euros and 34 for first class. Just doing a quick check at OeBB and it lists 63 for second class. Traveling in March. Any thoughts?

The 19 Euro fares are limited in number, so once they sell out, the next price level appears. You have waited quite late to grab the lowest fares, especially if your dates intersect with Easter. Either change your date of travel or look at trains at different times on that same day.

The main cities / capitals in Central and east central Europe are still connected by night train...Berlin, Krakow, Warsaw, Gdansk, Prague, Budapest, Vienna, Munich, I'm betting it'll stay that way.

I had read something on Seat 61 about still getting good deals days before but maybe it was only on trains at certain times. Anyways live and learn.
I did follow a piece of his advice which was to check both the Austrian and Czech sites and got the tickets much cheaper on the Czech site (as he noted one site could run out sooner than the other).
Excited! Traveling is wonderful!