I use vim-markdown. It's not perfect, but does what I want. Generates a table of contents in a side window, from there I can jump to any heading by putting the cursor on the item and pressing RETURN. The TOC is indented to show heading level, so it serves as an outline. In the main window there are commands to jump to next or previous heading, sibling, heading of current section. It has concealing for bold, italic, and (this one is important) links. It syntax colors code blocks using the syntax file for the language in the block.

I'm making small changes to it, and actually have my own fork on GitHub with a change that I needed. But the developer doesn't seem to tend to it much.