"Hi 1st-line -- I have an HX espresso machine (purchased from you).
I had some lingering problems with lime scale, so I took it to a local shop to do a thorough service (they do lots of E-61s but not Vibiemme in particular.) Trying to avoid shipping it across the country, basically.
Since then, when I pull water through the group head, a significant amount of it is coming out as steam. I didn't remember this from before, and it seems like it is too hot. I also thought this model had a "thermosyphon" that was supposed to regulate temperature.
Does this sound like an error in the repair, or something obvious to you?"
Limesale causes all sorts of problems before and after the repair. In your case, limescale probably slowed the flow of water even more than the restrictor in the thermosyphon loop and did not allow water to overheat the grouphead. Now, that the water is flowing correctly, the water is going back and forth quicker, which which you are starting to visually see.
On your particular machine, there is a restrictor on the thermosyphon loop, and it is pretty difficult to remove or even damage (other than clogging). It does slow down the flow of water. limescale would slow down the flow even more.
In our opinion, it does not sound like a repair error. The only thing is that if the HX (heat exchanger pipe) had welds corroded with limescale and the welds were removed during a descaling process, then the result would be a leak between the boiler and heat exchanger - a major problem that turns into a costly boiler replacement. If this was the case,
a) you would see the water overfilling the boiler and having no/little steam from the steam wand, and
b) eventually only water would exit the steam wand, and
c) there would be a low water pump pressure problem when extracting espresso.
Please keep in mind that not all heat exchanger machines have restrictors in the thermosyhpon loop.