will this be open weights?

I believe so. Openness has always been a consistent tradition of DeepSeek

I imagine this is based on their 'Thinking with Visual Primitives' paper, and they had mentioned that the weights would be released for that

This is something I would like to know as well.

But if not, does anybody know a recommended way to attach vision to deepseek flash (on a self-hosted infrastructure)?

Generally you just add a vision model as an MCP server like this: https://github.com/DavidEasden/opencode-vision

I would probably just give it a few days.

Deepseek is usually very good with open weights, they don't necessarily drop immediately, sometimes in a few hours, sometimes in a couple days.