In one word: Stale
I know having eaten many vanilla slices that a day old slice can easily maintain its freshness and pastry can still be crisp and flaky (in fact I had a day old slice from the French Lettuce this morning for breakfast after this disappointment!). Therefore, when I am trying to cut my way through a slice and the pastry just withers and dies I know that they are stale and ready for the bin, or at least a bargain-basement sale. I got so fed up trying to cut the pastry I actually threw away and just ate the rather average custard. Yes it had a few vanilla seeds in it, but slightly 'set' and bland would be an accurate description. Very sad.