What To Replace A Pair Of Lamm Ml2 Monoblocks With? Need More Power

Going from ML2s to a SS amp, would be quite the change. The Dartzeel mentioned or Pass Labs are the about the only ones I'd think might satisfy you. 
The more likely tube contenders come in many flavors. IMHO, the best high powered option for SET lovers would be OTLs, like the Atma-Sphere MA1 or MA2, the old Tenor 75s
Vandys are often paired with Audio Research; if headed that direction, the KT150 tube models are the best bet, a big improvement over KT120s or 6550 designs. 
BAT 6C33 amps would work, but guessing the combo would be run a bit on the dark side.
Manley & VTL are usually in the conversation when talking powerful tube amps, maybe someone with experience using them w/the 5As could chime in. Cheers.
Spencer

First, let’s be clear the Lamm amps are special. Vandersteen are designed to be an easy load. I have owned the 5a in big rooms, one 18’ x 40’ w 10’ ceilings using a 200 wpc Ayre VX-R. In smaller rooms have used something as small as 75 wpc ARC REF75se, I don’t listen super loud. In SS on a budget, I would lean to Ayre and the wonderful Aesthetix Atlas stereo, of course there are others. In Tubes ARC 150, VTL 185 and even the Quicksilver 120. The right Music Reference also would be fantastic. Jim

One aspect of the Quicksilver Mono 120 that’s intriguing is the very simpler/straightforward circuit (Only 2 output tubes per channel) which is I believe sonically advantageous (And simplicity is a strength of SET amplifiers). I could imagine this amplifier ’potentially’ mimicking some qualities of the Lamm ML2 relative to the transistor amplifier recommendations. @nightfall acknowledged, he trying to lose as little as possible the special  character  of the Lamm.
Charles

Jadis JA80Mk2 or JA200Mk2 would be a good match to Vandy 5's. 

Vandersteens are awesome but they need some power to be at their best. The fact those 5as have a powered bass module, means the absolute power required is no where near as high as it could be. 
You have a few options.  The room is large but not crazy and those aren’t difficult to drive.  
You could look at something like a BAT Rex 3.  Two issue with BAT is everything needs to be BAT or it loses a lot in terms of sound quality so you would probably want a BAT pre as well.  They use the same tube but it is also a parallel SET design which could be problematic.  SETs don’t like being driven hard.  One upside is you can get this on a 60 day demo from Music Direct.  My hunch is a 45w push pull will outperform this when driven hard but I mention because of the tube type.  
Another really interesting option is Jadis.  It is a KT120 based solution but is going to have oodles of power.  They have a 150w class A/B amp that is $16,500.  They have some high powered Class A Monos that are in the $30k to $40k range.  Gorgeous equipment and a sound that is somewhat legendary.  

Angstrom is an Italian brand and the Stella line of products is incredible.  The SSA 100 or the SMA 150 would be A+ choice.  $17k and $32K respectively.  It is Class A, zero feedback and uses EL34s so it has a very appealing sound.

The final (and IMO best) option would be the KR Kronzilla DXL Monos.  They are 140w push pulls using the Kronzilla tube and are also Class A with zero feedback.  These will be closest in terms of sound to your Lamms I think other than the BAT, but the BAT seriously might struggle.  
I am a KR dealer and am getting ready to sign up for Jadis and Angstrom as they are awesome lines.  I have no relationship with BAT.