Greg is quite right.   No single piece of the jigsaw will ever give a complete picture, and the fact that de Vere was an accomplished poet does not mean he continued writing after 1570.

But there are lots and lots of pieces that seem to fit a certain pattern.

For example, the Geneva Bible is not in itself conclusive.   (the margins of the bible, owned by the de Vere family, were annotated in its margins prior to 1560 with several dozen phrases that later turned up in Shakespeare's plays).   If nothing else, it does seem that our Bill had the opportunity to read it, otherwise the phrases that do match came from another remarkable set of coincidences.   

Another f'rinstance. As a very young man, De Vere adopted a coat of arms in which a lion is shaking a spear.   Sheer happenchance that the second editions of the early plays are attributed to "Shake-Spear" (goddamit - our Bill couldn't even spell his own name correctly the first time round).   The first editions were not signed by anybody.

And so on, and so on, and so on.

(That's the problem with us stupid conspiracy-theory fanatics - there is so much stuff around to encourage us, that we keep making these wild speculations.)