There is a somewhat disconcerting presumption in this thread that Vietnam was such a horrible experience that all veterans came back emotionally disfigured or in other ways darker and more negative in outlook. That's several million people. I'm of that generation (although not a veteran) and I know several friends who went through the grinder apparently unaffected.
Nonetheless, I suspect Star Wars would have never been created in the form we saw it even if he did go into SF and fantasy. From what I've read about Lucas, much of what eventually went into Star Wars was the product of a long period or relatively uninterrupted peaceful "childhood" extending from Lucas's teenage years through his time at USC into the early 1970s. rebellious kids, hot rods, etc. Break that up with a year long tour in Vietnam and Lucas would certainly have another life perspective to work with if he still pursued film making. Whether that would make him seek to convey these experiences in film or avoid them in fantasy is only speculative. A lot would depend on if he still made the connections with Spielberg and Coppola he did in OTL and had the early success to switch from directing his own films to producing.
But for funsies, here is a possible alternate Vietnam Vet Lucas directorial filmography:
THX-1138 (1971) Visually and stylistically similar to OTL, but in this version the underground society is waging a war with an unknown enemy. The drug-induced torpor that people live in is designed to make people forget that regularly the most fit members of society are removed and further engineered to be soldiers and utilized until "expended" in the war. After removing his drugs, the protagonist and his few friends discover this and seek to expose the truth. They are eventually all caught, killed, and society continues.
American Graffiti (1973) This version has two parts. The first part is similar in tone and content to the OTL movie, but shorter. The second part expands on the OTL "what happened to them" epilogue and follows the main characters via set pieces several years later. Several die in Vietnam, others become disillusioned, unhappily married, etc. The film is a surprising success.
Starship Troopers (1976) Lucas makes the Heinlein novel into a movie. He makes a straight adaptation without the obvious sarcasm of the OTL film. The civilian Federation culture Rico and his friends come from is blander and darker and Federal Service is portrayed as the way out of this stagnation into freedom. Plot and characters are truer to the original with sexual division of labor in the military, attacks on Skinnies as well as bugs, and the power suits. Like OTL Star Wars, it is a risky investment in a major special effects SF film, but in tenor it is more suited to the pessimism of the early-mid 1970's. The film features a few major actors in strong supporting roles. It is moderately successful and a qualified critical success, but is far overshadowed by Spielberg's "Close Encounters of the Third Kind" released in the following year and which sets the direction for mainstream Hollywood toward more uplifting and lighter fare.
Steel Hearts (1978) The first complete divergence from OTL. Lucas makes a movie about stock car racing. Protagonist, Luke, is a middle-class California kid living with his aunt and uncle. Luke is arrested for drug use, imprisoned and eventually released from prison in Alabama. He's a car nut and eventually joins the pit crew of an aging, but still top-form driver, Obie Canoby, who also teaches him his driving skills. They begin winning races and eventually come up against Garth Ray, Canoby's former friend but now sworn enemy. In the penultimate race, Ray deliberately causes a crash that kills Canoby. Luke then confronts Ray and this leads to a massive race/car chase through rural Alabama. Ray swerves to miss a school bus and crashes. Luke pulls him out of the car. As Luke and Ray talks before Ray dies in Luke's arms, it becomes apparent that Ray was Luke's long-lost father. The film was critically appreciated, but a box-office failure.
Paths not Taken (1982) A very dark comedy based on time travel. Marty, a middle-class California teenager living in a broken home has a severe drug OD and wakes up in 1955. He ends up meeting his future parents, and out of spite, works to make their lives miserable. His dad drops out of school and Marty ends up falling love with and marrying his future mom. His dad goes on to be a powerful Reagen-esque politician, whose story we follow only through newscasts. In 1980, dad is elected President and in 1982 his militancy leads to a war with the USSR. Marty is now divorced from his mom and are fighting over custody of their handicapped child when a nuke hits Sacramento and they all die. He wakes up in a hospital bed in the original TL, sees both his parents standing over him, mouths "I love you" and then dies. Film was a critical and box office bomb.
Lucas never directed another feature film. He was approached about doing a Star Trek film, but Gene Roddenberry nixed the idea. He wrote a few SF novels, including a sequel to Starship Troopers, authorized by the Heinlein estate. He eventually moved into the Ted Turner cable TV empire, and in 1988 became host and chief consultant for Turner-Lucas Classic Movies, a TBS series focusing on classic Hollywood movies of the 1930's-1960's. Lucas died on July 12, 2001 of a massive heart attack.