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.
This I2V workflow is based on my earlier Wan Join Clips workflow. This simplified version is designed to extend a video using Wan with Vace based on a single beginning frame. Yes, I know Wan Image-to-Video already exists, but the results you get from using the Vace system can be quite different, so I think this is a useful tool.
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 initial image (and reference image, if you like), 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.
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 or the interpolation settings, 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.