duh, I knew that...
(I should clarify that this person has explicit permission to point these kinds of things out to me; it's part of how I will, I hope, get better at Hebrew. She didn't do anything wrong here, so don't get mad at her for picking on me or anything like that.)
Now when I am sitting down and slowly reading some Hebrew text, I can (usually) spot the roots and interpret them. (More often than not I can't in spoken Hebrew, alas.) When I was first looking at this portion I certainly recognized "v'samachta" as "you will rejoice"; I now always try to do ny own translation before consulting a correct one. Of course, I had the vowels and other marks then, including the dot that turns "shin" into "sin". But I've seen some of these words without vowels before (like in "simchat torah", the name of an upcoming holiday). I don't think I needed the vowels so much as I needed to be paying more attention while reading at speed.
So I think the problem is me -- my reading style, or my level of attentiveness. At times I'll be reading text (usually during services) and a word will jump out at me, completely unsolicited, because I recognized the root without thinking about it; I've commented before about how sometimes the subprocess that produces that outcome is a distraction. :-) And yeah, I don't want distractions during a torah reading, but if it had happened during any of the dozens of times I practiced this passage sans vowels at home, it probably would have stuck. So I'm left wondering what changes I need to make in my reading, learning, or leining practices to increase the likelihood that I'll make these kinds of connections earlier.
It's one thing to not know a word because I don't yet have the vocabulary. It's quite another -- and frustrating -- to know a word and not recognize it in the wild.