Troublesome apostrophe.

In the book "Eats Shoots and Leaves", Lynn Truss raises the example of "two weeks vacation" (1), which, she argues, should be "two weeks' vacation". She is adamant that there ought to be an apostrophe after the "s" of "weeks".
But is she right? There are two reasons why I doubt what she says :-
1. This is not an example of the possessive case. The vacation does **** to the two weeks. At most, there is an adjectival association between "vacation" and "two weeks". Does an adjectival association merit the use of an apostrophe? It seems to me that a logical extension of what Lynn Truss has said would be the assertion that "lorry driver" is grammatically incorrect, and that we should say "lorry's driver". "Two weeks" is to "lorry", as "vacation" is to "driver". (Brings back memories of my eleven-plus examination).
2. There are many examples of situations where the similar use of anapostrophe seems to be incorrect. Although I would write "Einstein's Theory", as soon as I include the definite article I would want to write "the Einstein Theory", without the apostrophe-s. For example, I might write "The experiment was designed to test the Einstein Theory of Gravitation". There is a much greater degree of possession between Einstein and his theory than there is between "two weeks" and "vacation", yet "the Einstein Theory" seems correct without an apostrophe. At any rate, it seems correct to me.

I met a similar problem a few days ago, when I was the only respondent to Matti Lamprhey's posting "Contain Nuts". In that reply, I was unsure whether to write "Ames Room" or "Ames' Room".
(1) Or something similar. The book is now back in the library, and I cannot refer to it.
Richard Chambers Leeds UK.