As far as I know about IT - which is fairly rudimnentary - updates and updated versions INCLUDE that which was part of the same Version. Here V5 > V5a > V5b > V5c > . . . would all roll forward into . . [ drum roll .. ] Tarah! V5d . . a new version would be V6 > V7 > V8 . Updates temnd to "mop-up" issues from the previous update number. Version numbers tend to have some truly radical new functions that are way beyond that which preceeded. One of the biggest to assist us and Sony was to create V5 which took it out of the reach of WinMisery and 98se etc .. and straight into the arms of XP.
There was a "C" update, it was for Markers being retained in MPEGs, no jump from B to D. Read the white papaer on the incremental versions, it is there.
Each "update" is actually a FULL version of the application. So if you download Vegas 5 C you would have every correction and improvement up to 5.C. If you upgrade to D, that includes everything new/changed in D plus what was added/changed in C and so on.