The contour is a powerful tool for filling in neighboring voxels of similar contrast. You can think of it like a fill (paint-bucket) tool, but with the added benefit of being able to adjust fill thresholds on the fly and preview the outcomes live. The following example shows how the contour tool can be used to quickly fill in a vessel.
Open the Voxel Edit window, then select the Contour option, select the value you would like to fill with.
Hold ctrl-alt-left click in the viewport, and move the cursor up and down until the area you want to fill is surrounded by a white line, make sure it is an isolated island as everything connected to that island will be filled next step. This controls a lot like windowing with the cursor does.
Hold ctrl and left click inside the island you want to fill. You are done!