I have not worked on the VB
-2, but from looking at the manual, it appears that once you remove the back, it would be possible to reach around the power tubes and get to the two preamp tubes.
Worst case. you remove a couple of the power tubes. They pull straight out and re-insert just as easily.
edit: Looking at a picture of the back, looks like there are 4 screws to remove the back grill. Then all of the tubes are accessible.
edit again: You may have to pull two of the power tubes to reach around the transformer. Disregard the small tube immediately in front of the power tubes, that's the phase inverter. The two preamp tubes have tube shields around them: rotate the shields slightly and the shields pull off. Then the tubes pull straight up.