The bigger problem facing you is that features and associated standards for switches are evolving so rapidly that protecting your investment will be a problem. Vendors take a couple of different approaches to this kind of protection. They may make all the components of the switch completely replaceable, so the investment you're protecting is little more than the case and power supply. Or they may put in a few powerful general-purpose CPUs and memory, so their product can handle any standard you'd want to throw at it in the future. With the former approach, you could find yourself financially ahead by not bothering to reuse trivial par ts of the box you purchased.
Some products attempt to strike a balance.
Whether they do or do not strike the right balance is the issue left to you as the buyer. Most of us just want VLAN technology that works and allows us to build a more scalable network. Interoperability is not the primary issue. The technology is new enough and the standards are complex enough that a one-vendor solution is still the best way to go and will be for quite some time to come. While features like virtual LANs are important, so are management features.