Maybe it is the stainless needles that cause the issues vibrating, but it takes a lot to get this stuff flowing. If you ever go the castable route report back how the vibration casting goes, I'd be curious to hear another persons experience. I'm confident of the strength of my reciprocating saw, how do you use the saw to apply the vibration?
On to the Refractory. From my understanding talking with the staff at H-W when I'm there it sounds like their products are used by all the major industries here in Detroit. In my personal experience the other day I jumped up and down on a 1" thick arch I cast in 2011 WITHOUT stainless needles in it, and it didn't break.
I choose to use KS4V-Plus for multiple reasons. H-W always has stock on hand. It is a lower duty rating then a lot of their stuff, but far above what we need for pizza ovens, and it is by far the cheapest dense castable I have found. In small quantities I pay $0.57 a pound. My oldest castings have been in use since 2011, have rolled around on casters, were heat tempered imprecisely in place, been through brutal Michigan winters exposed to the elements and still don't have a single crack in them.
As I've mentioned a few times here I use a high percentage 409 stainless needles that I buy from Hi-Temp on ebay. None of my local suppliers stock them but high temp sells at a good price and can fit 10 pounds in a flat rate box. I use 5% by weight of the dry castable. KS4 has a yield of 125 lbs/cubic foot.