It was not possible to use both real kinds in a single fortran subroutine on the VAX. The fp format was selected with a compiler option. Presumably if the VAX had ever supported an f90 compiler, that would have opened up the architecture. Ironically, DEC fought against f8x, and eventually f90, tooth and nail, despite the fact that the language would have ideally supported their hardware.
Regarding IEEE arithmetic, the standard was published in 1985, but that is not when everyone switched. There were new machines being built at that time that still did not use IEEE fp format. New VAX systems were designed and sold up until about 1990, all using the same fp formats (32-bit, two different 64-bit, and 128-bit). To give another example, the Cray 2 was first built in 1985 using its own fp format, and it was the fastest computer in the world at that time. The Cray Y-MP followed in 1988 and the Cray C90 followed in 1991, all with the Cray fp format. I think they sold C90s up until 1996. Of course, both DEC and Cray were also selling other machines that did use IEEE arithmetic in the 1990s, but I’m just pointing out that other fp formats were still common well after f90 and even f95 were introduced.