Soju is not a weaker vodka; I can assure you of that.
Except, it generally is. Most vodka has an ABV of about 40%. Soju on the other hand has a usual ABV in the 16-25% range. Some can be higher, but we're talking normal products here. There is stronger Soju just like there's stronger Vodka, in general though, it is weaker.
Soju ranges from 12%-54%. Plus, soju is not well-distilled; it has impurities that vodka does not.
Edited: We're likely both right. I was in Korea in 1999; back then the alcohol ABV values were quite a bit higher than today. I am also going to guess that the soju ABV content was skewed quite high around the Army bases. For reasons. I also could be wrong. It's been known to happen.
Soju is to Korea as sake is to Japan. OP mentioned Korea so it's unlikely a mistake. And Sake is a similar rage for abv. It's naturally brewed and not distilled. Most yeast dies around 20% abv so virtually no naturally brewed product can get stronger. Vodka and other spirits are distilled so the skys the limit. Most are 40% for historic tax reasons.
Not sure why you wanted to give me a geography lesson when I obviously know what it is. Soju is far stronger than sake. It may be fortified in some cases.
Because the way your original comment is worded it can be understood in two ways: either the soju or the vodka was written wrong. You meant the vodka, they thought you meant the soju.