To blend two materials using Vertex Paint, start by ensuring your mesh has a high vertex count—Vertex Paint needs a dense mesh to work well. In the Material Properties, use one material slot and include both of your material setups, each ending with a Shader node (copy and paste them from their original node setups). It's best to group each setup (Ctrl+G) for clarity. Add a Mix Shader node to combine the two materials. Then, add a Color Attribute node, plug it to the Factor input of the Mix Shader node, and set its name to match your Vertex Paint layer (usually called Attribute by default). Switch to Vertex Paint mode in the 3D Viewport to paint. Stick to the black and white colors (hold CTRL to paint with the second color): this controls controls which material shows where. You can always swap the materials with one-another in the Shader nodes, by selecting the Mix Shader node and pressing Alt+S. If scales are off, make sure to Apply Scale, and tweak the Scale value in the Mapping Node of each of your Material groups.