I never found a way to convert the __m128 back to an FVector, but by examining the value with a watch, I was able to find the data.
The __m128 appears to be a generic structure which just… contains 128 bytes of data in various configurations for reasons I don’t understand, in the form of multiple arrays which store various basic data types of different sizes.
I found the Z-value of the vector here: Translation.m128_f32[2]