In 1948, the US lost all trade with China, but no one even thought of going to war over this.
Actually there was a lot of thought. We did send a lot of military material and other aid to the KMT government post 1945. Ultimately Trumans government had to make some tough choices and investment in rebuilding Europe was chosen over another war in collapsed China. Note how in the 1930s Japan had supposed China as a trading partner with the US. The former had been stagnated for decades & was close to 'failed state' status in current terminology.
Has there ever been a time when the US imported any rice from the Mekong? ...
It was part of a global trade network the US made a lot of wealth from. Germany and Japans efforts to create economic control over large regions & were badly disrupting that trade/cash flow that benefitted the US. The rice market was upset much like the the Japanese taking control of the Michelin rubber plantations. The US has never been a autarky. Even in colonial times the coastal settlements were heavily dependent on international trade. Over two centuries roughly two thirds of the US economy has depended on international or global trade & been tied to the free trade policies. One of the things that aggravated the Great Depression in the US was a post Great War reduction in unrestricted trade. On the surface the war exports to Britain, France, and a lesser extent smaller nations during 1939-41 hid the damage to the US economy as Germany & Japan turned the former global markets and trading system into wreckage. As the Interventionists saw it in 1941, were the Axis to continue there course the US would ultimately be economically limited to the western hemisphere & its former trade with industrial Europe or the prosperous China of the 19th Century reduced to a joke.
It must also be understood the intent of the Embargos responding to the FIC occupations were not intended to lead to war. It was expected the sanctions would force Japan to negotiate a settlement that would reduce its threat & eventually settle the China war. The risk was a bit underestimated by Roosevelt's group, and Congress in general. By the time the reality was realized Japans leaders had already grown desperate and made their decisions for war. When negotiations collapsed neither side had any clear or realistic idea how to avoid war.
If Mekong rice was vital to the United States, why didn't we fight the Vietnam war like world war II?
The US did deploy a army at similar density as in WWII, & did indulge in aircraft bombing at a scale larger than WWII. Viet Nam or SE Asia was just one region or theatre in WWII terms & it drew resources at that scale or larger in some items.