calling vandersteen 5a owners

The few times this has happened in my system I haven't noticed anything major. Sound wise I'd say play and have fun! Since I never listen about 90db or so it's not an issue for me.

It takes the subwoofers in a Model 5 or 5A about 4 hours to rise to the midpoint of their excursion. They will play prior to that, but you want to keep the levels relatively low until they are ready.

I've not faced this situation. However, my guess is that once the sub is broken in, and especially the cones loosened up, it wouldn't take but a few hours to get back to snuff.
You might try some thing like the Granite Audio "Extreme System Workout & Fine-Tune Burn-In CD". This has a set of tracks that will give your whole system a workout. In particular, it has three sections so you can work out your bass, midrange, and treble separately. The bass tracks include 2-160Hz pink noise, 0.5 sec. sweep tones, and white noise. Midrange has the same 3 for 106-2560Hz, and treble has the same 3 for 2560-20KHz.
I have found that running this occasionally seems to help keep the whole system more alive.