Nitpick, but DEC never made a mainframe. Their products like the PDP-11 were considered minicomputers (even though the CPU was the size of a fridge) to distinguish them from IBM’s mainframes and medium sized computers.
But the PDP-10 and VAX 9000 were basically mainframes. Million dollars or more. Whole large room with three phase power. Standard building AC might suffice but that was pushing the margin. And the faster clocked VAX 9000 was water cooled! That's not a minicomputer.
Why is that not a minicomputer. From our perspective it's a massive installation; from the perspective of the time, it was not necessarily.
Really there is no good formal definition, dividing computers into three groups(mainframe, minicomputer, microcomputer) based on how much you payed for it, is as good a mechanism for figuring this out as any other.
However the VAX 9000 wasn't. It was initially designed to be water cooled but during development they improved the air cooling enough that water cooling was never shipped.
One VAX 9000 CPU did around 70k Dhrystones/sec, so 40 VAX MIPS (DMIPS as we call them now).
A modern simple RISC 5-stage in-order CPU (e.g. Berkeley RISC-V Rocket) with decent branch prediction does 1.6 DMIPS/MHz, so will need 25 MHz to match the VAX 9000. However the December 2016 HiFive1 ran at 320 MHz, so would be around a dozen times faster than the VAX 9000 -- but without FPU or MMU. Today, a $5 Milk-V Duo running at 1 GHz (64 bit, and with MMU and FPU and vector unit) will be 40x faster than a VAX 9000.
The VAX 9000 has a vector unit, capable of a peak 125 MFLOPS. The Duo's C906 VFMACC.VV instruction has 4 cycles latency, for two 64-bit multiply-adds or four 32-bit multiply-adds, so at 1 GHz that's 1 GFLOP double precision or 2 GFLOP single precision, 8x a VAX 9000.
Don't even ask what a modern i9 or Ryzen can do!
The 750 was later technology, IIRC MOSFET. It also lacked the 780’s “compatibility mode”, which allowed VMS to run 16 bit RSX-11 executables by implementing the PDP-11 instruction set in addition to the 32 bit VAX one, and could boot without a console processor. If you didn’t need all the users on one machine, the 750 was cheaper per user.
Assuming, of course, that those "medium sized computers" are the midrange.
Ultimately, within IBM at least, "mainframe" ended up just referring to any computer that implemented the System/360 architecture or its descendants.
Outside IBM, I've seen the term applied to just about any multiuser system accessed primarily through a terminal interface, including x86 servers running Linux, though typically by nontechnical users.