Depression caused by loneliness has been the biggest issue older Vietnamese expats have had to deal with for several decades now. Even when living in a Vietnamese community, making friends is not easy for many older folks. Finding a job when one doesn't speak the language and not familiar with the customs is an almost impossible task.
I truly feel for you and the problem your father is facing, but I don't think he will be able to compete in the American workforce. Within the Vietnamese community in the States, the kind of job that is readily available to people who don't have the language and specific skills would only be service related, but not as frontline staff. They're utilized mostly in back room (kitchen staff in restaurants, stock clerks in grocery stores, general factotum in shops and offices, etc.) Often, they're paid less than minimum wage (which at the moment is between $7.25 and $10.50/hr, depending on the state) and more often than not, they aren't treated very fairly.
You asked about activities that lift an older person's spririt. Older Vietnamese who don't need to work often hang out at specific coffee shops to shoot the breeze with other retirees, volunteer at churches or Buddhist temples, teaching Sunday schools to Vietnamese children, participate in Vietnamese civic organisations, etc. None of those would suit your father, since based on what you told us, he's not in the US to enjoy his retirement years.
I don't know if returning home is an option for him, but I would wholeheartdly recommend that. In VN, he at least has his friends, his family, his affinity with football, his familiarity with the customs, his throughout knowledge with the ins and outs of society. In the States, he has to start a new life with many disadvantages and without much support from his relatives. Easy to see which picture is more pleasing to the eyes.
For most people, finance is certainly tighter in VN than in the US, but for newcomers (especially the older ones), to live and work in the States, the trade-off for more money can be intolerable sometimes.
As for your brother, his priority would be the language. He needs to speak English fluently first before he could find a decent job. After living in CA for one year (with proof of residency -- rent, utility bills, ID/driver license -- he can enroll in a community college and pay the lower in-state tuition. He should concentrate on English classes plus the prerequisites. A degree may or may not be needed for decent employment, but no decent job can be had when the applicant is not fluent in English.