I’m trying to join two separate vectors into one so that I can fill in. I can combine the vectors by flattening it but it won’t let me fill in the enclosed vectors. When I go to Menu > Vectors, I see the “Join selections” button but it’s disabled and I’m not sure in which instance I can use this function.
Join selections simply joins two selected vector points with a line. What you need in your situation is a bucket tool (B). Click the areas you want to fill with it and they will be filled with whatever fill you have applied to the current layer.