IMPORTANT: There appears to be a bug in Version 2 of this workflow. Something about the desaturation logic is behaving strangely, related to the node logic that allows you to skip that process if you want. Until I can release a fix, I recommend using Version 1 instead.
Important credit for this workflow goes to pftq for creating the seamless video extend workflow. That workflow was the first to show me this feature existed and gave the building blocks I used to make this version. The workflow by pftq uses Kijai's Wan Wrapper nodes, so if you prefer to work with the wrapper nodes in order to make use of all the options and add-ons, then by all means give pftq's workflow a try. This workflow uses all native ComfyUI nodes for the Wan processing.
One of my goals with this workflow was to automate the process as much as possible. In order to get Vace to fill in gaps, you must first create two custom videos with masking in the correct places. This is painstaking to do without nice editing software, but the simple extension masks are something ComfyUI can accomplish with the help of a few custom nodes. This workflow is the result of that effort.
The workflow looks large, but once you've made sure all your models are loading correctly and your video save paths are set the way you like, you should only need to use the three groups across the top-left. You set your beginning clip, you set other values like steps and so on, then you run the workflow and check your output. All of the masking, resizing, and frame interpolation/smoothing is automated.
This extend video workflow is based on my earlier Wan Join Clips workflow. This simplified version is designed to extend a video using Wan with Vace starting with a beginning clip.
All of the machinery is exposed in all the other groups below, so feel free to mess with anything you like. You will find a few settings buried inside that you may want to tweak, like the Model Sampling Shift value, for example.
On my machine, using the default values given here, my 16gb VRAM card can load and run this workflow without using any shared GPU memory and without block swapping. It runs at a steady 14.2gb, usually, though your mileage may vary.
I hope you enjoy.