If this is something which you do (or have been told to do) then I would love to have you on my next flight.

Reality check time, and after literally hundreds on LH flights (in CW and F), and more than a decade of being at least a GCH each year - the VLML is most definitely not the same as the ones on the CW menu.

Both your starter and the main course are the specially loaded VLML options (which are not infrequently the same as VGML and AVML, and occasionally have 2 or 3 of those identifiers on the sticker on the bottom/side of the tray).

You cannot automatically get any of the starters or the main courses, even if they are vegetarian - as you are not "entitled" to a regular meal by requesting the special meal. It means that potentially one less regular meal was loaded and your "switching" would deprive the normal people of their selections.

If you happen upon a friendly FA or pursar, they may let you switch to a "normal" meal and allow you to select the vegetarian option from the menu (for either the starter or the main course or both) - from the get go.

More often, they ask you to wait until everyone else has selected and/or been served, and only then, if there are any remaining items, you may be able to have those (again, keep in mind that by doing this, you reduce the selection of the FAs who take the unwanted CW dishes - so unless they don't mind or are eager to help you, this may not even be presented to you as an option).

In some cases, you will be actually given a stern lecture about how selfish you are to have ordered a special meal when you really wanted a "regular" meal item and by doing so, you are not entitled to anything other than what is marked VLML (not even a regular pudding/cake for dessert, you can have the plastic-wrapped fruit pieces or nothing at all).
Yes, out of a few hundred flights I have had that unfortunate scolding at least a dozen times.
:-(