It's generally because of the custard filling- if you are talking about the sort of vanilla slice which is a thick sliceable custard sandwiched between sheets of puff pastry. The yolks are generally to blame, as they contain an enzyme which breaks down starch. This enzyme is usually deactivated when the mixture boils- but that's not something ideal for a custard, so that's usually cooked at a slightly lower temp. So some of those enzymes remain. As the days pass, your custard will begin to dissolve, making your slice a slush. If you use an egg free custard powder, like Bird's, you can get around this- but of course, it's not the unctuous vanilla custard we all love. Sorry, but vanilla slice is one of those things that just isn't meant to hang about long. If you find one that does, it generally isn't that tasty.
I'm guessing it's somewhat like other baked goods with few or no preservatives, and best soon after baked. That said, are you storing it at room temperature? Maybe put it in the fridge for keeping as long as 1-2 weeks. See this chart for tips... http://www.eatbydate.com/grains/baked-goods/pie-shelf-life-expiration-date/
OK, Brownwyn. I go back to the chemistry of the baking. And maybe vanilla slice is more like a croissant, which is wonderful the first few hours, tolerable at end of a day, needs warming on 2nd day, hopeless on 3rd. I'll admit the chart looked optimistic (or seemed to give maximum duration) to me. Sorry it wasn't more use.
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That said, are you storing it at room temperature?
Maybe put it in the fridge for keeping as long as 1-2 weeks.
See this chart for tips...
http://www.eatbydate.com/grains/baked-goods/pie-shelf-life-expiration-date/
I'll admit the chart looked optimistic (or seemed to give maximum duration) to me.
Sorry it wasn't more use.