How old is the usage of 蔬菜 to mean vegetables? Did it used to be only 蔬？Or only 菜？ I can't find any other usage of 蔬 (other than things like 果蔬， where 蔬 seems to be just a stand in for 蔬菜）, whereas 菜 is a much more productive and free morpheme. What meaning can 蔬 be said to have by itself?
Looks like the earliest use of 疏菜 can be traced back to "疏 - 周礼太宰注 疏菜也". https://baike.baidu.com/item/%E8%94%AC/6708649