1m/s ascent speed is fine, the problem with VWT dives is that you tend to rocket up in the last 10m or so. It's possible to do 2m/s quite easily if you get carried away and keep kicking. This isn't too bad for a couple of dives but if you're doing several to, say, 40-50m the fast ascents increase the risk of DCS. Not that anybody knows exactly what depths/repetitions/ascent speeds would put you at high risk; rather it's just a general caution.