While I like water based acrylics like Vallejo for brush painting, they dry fast.
I've just started using an artists acrylic 'wet palette' and my Vallejo paints stay useable for hours. If you've never seen one before, a wet palette is a paint palette consisting of a tray and then 2 layers of paper. A thick, absorbent sheet and a thin sheet of paper that will wet but not let paint bleed through.
The thick sheet is placed in the tray and soaked with water. The thin sheet is placed on top. The thick sheet acts as a 'reservoir' to keep the top sheet wet for hours (or even days). Waterbased acrylics can then be squeezed out onto the wet palette but don't dry out. Nor do they thin out... they remain perfectly brushable. A wet palette can be used like any palette for mixing colours and is great for shading where you might want 3 or 4 colours on your palette at the same time.