That use to be the case but with todays technology in plastics and plastic/vinyl care products, it's no longer an issue. I recently read a post from another board, rather that retype it, I'll just quote his findings. Either way, I don't think you have anything to worry about anymore. Hope this helps.
"First of all, modern plastic dashes, (this includes vinyl, which is a type of plastic), are far more advanced in their chemistry than vinyl dashes 30, 20 and even 10 years ago. Just like most things, when new technology comes along to make a product better, most companies adopt it. I can?t say that I have seen too many dashes on modern cars crack.
The way older vinyl dashes were made, a Plasticizer was included in the vinyl to help make the plastic soft, elastic and flexible. Over time, and exposure to heat and direct sunlight, the plasticizers in the vinyl break down, deteriorate and the vinyl becomes hard and brittle and susceptible to cracking instead of flexing.
Some vinyl protectants in the past used a plasticizer in their formula to soften the vinyl to make it flexible, and keep it flexible. Here was the problem with that, there?s an old saying, ?Like products dissolve like products?, this was the case with the plasticizer in some protectant products, it had a tendency to dissolve and leach out the vinyl?s original plasticizers prematurely thus accelerating the deterioration process. This could lead to cracking in vinyl dashes because the vinyl became more brittle and hard.
An analogy is your finger nails. When your fingernails are hard and brittle, they will crack. After exposing them to water for an extended period of time, shower, swimming, bath, hot tub, etc. your fingernails become soft and flexible, much less prone to cracking or breaking. Because of new technology used to make plastics, plasticizers in products are no longer an issue."