Model the manhole cover as geometry? Use a projector to make a decal? I wouldn't use vertex blending for this as it needlessly complicates the shader and isn't very flexible.
Not totally following you but here goes... Tile your tiling map over the entire surface. Locate the approximate borders of that original texture (one tile square) in the area that you want the manhole cover. Cut this area out. Duplicate your original tiling texture. Paint a manhole cover in the middle, but stay away from…