Looking back at what we did, I wrote about 2 pages, split into 2 sections.
I think I detailed a rough timeline of the past 2 years (which is the period that they care about in K1 for the most part). As we lived in different countries at the time, I basically detailed all the times we met since then (e.g. last time we met I proposed, before that we met to travel around country X, before that we met to travel around country Y, etc). I wrote where we went, the approximate dates (based on passport entry stamps), and roughly what we did/why we went (generally tourism and to see family), but didn't go into much more detail than that. I think I also had like 1 or 2 pictures from each trip I mentioned, but not a ton. I'd fit at least 2 pictures per page, 3 might fit as well (just pasted them into the Word document with a small caption).
Additionally, I had a paragraph about how we met and how long we've been together.
Then, if they still cared to read, I had another section briefly summarizing our relationship between 2 years prior and the X years prior to that since we first met.
In short, I tried to keep it brief, and so it was about 2 pages of text, 1 page for 2 years prior to the I-129F, 1 page for beyond 2 years if they cared, and then separately photos from any trips I cared to document. I think it was probably a bit much for the consulate we went through (not Vietnam), but I think something like this would be a good balance between focusing on what they care about but giving a bit more background if they want it, as I felt in general Immigration wants to understand the overall facts and story of your relationship, not every minute detail.
I'd bring it to the interview unless there are rules not to bring extra stuff for some reason, but maybe condense it a bit as I agree with others that no one wants to read 40 pages. Unless you have been in a really long and complicated relationship they don't need to see everything, just enough to get the sense that the relationship is real / valid for the immigration benefit that you are applying for (although I also agree, cross-post it in Vietnam sections to see if there is country-specific advice on quantity/quality of materials that you need to bring/submit).