How to stabilize speakers without spikes.

you can still use spikes but you will need to get either the support discs or Target Spike Shoes. Support discs from Music Girect are all metal, they are affordable and do exactly what they supposed to besides there is no sound degradation. Target Spike Shoes that Cable Company sells are also affordable. They are metal on top but rubber on the bottom. I have spike shoes under my SolidSteel 5.4 component rack and Support Discs under my speakers. And use spikes without any damage to the floor.

Herbies Big Fat Black Dots are the ticket IME. Perhaps some type of shim could be used to tilt these.

After living with carpet over wood floors and spikes on my speakers for many years, my wife wants to have a wood floor (maybe tile) installed in our house

This will change the tonal balance of yr spkrs' response (usually upward). You'll get more mid-hi frequency reflections off the floor. This simply means that you probably have to change the spkr placement; you may get a more "airy", "detailed" (if slightly bass-light) sound -- and like it more.

As to the spikes you're using: you can continue using them by placing hard metal disks under the spikes (a nickel as suggested) -- but the result won't be exactly the same. Until now you've been coupling the spkrs to the floor mass... the metal disk will decouple them.

I strongly suggest you buy nothing and experiment with whatever you find at home -- or, in extremis -- just go to home depot & buy 3 vibration control puks.

Take one of your Vandersteen spikes to the local hardware store and purchase either hex or carriage bolts with the same thread size. They won't harm the floor and will provide solid contact and can be adjustable for desired tilt. This will cost less than $5. Another option is to find similar products from an audiophile web site and pay $100+ for similar, but much fancier, product.
The hardware store option worked great on my Meadowlark Shearwater HRs.

I recommend you try Soundcare Superspikes (http://www.soundcare.no) and they won't scratch your wooden floors.