I went through the same thing 2 months ago for a Dec trip to Vietnam. If you are booked in economy, SQ does not assign seats. They will take down your preference and from what I can tell they do try and accomodate you. However, you could be stuck with what you get. I had no luck even as a *gold to get them to assign seats ahead of time. I even spoke to a supervisor to no avail.

On our return journey, we were flying NRT-LAX and got put in the last 2 rows of economy - there were 4 of us and I had asked for the window seats. The seats turned out to be fine - still some distance to the lavs behind and they had extra space at the side due to the curvature of the aircraft. The only downside was when deplaning...