Defense Secretary Caspar W. Weinberger and Gen. John W. Vessey Jr., chairman of the Joint Chiefs of Staff, said yesterday that the armed services could fill their ranks without the help of proposed legislation requiring that all young Americans perform some form of national service.

Sen. Gary Hart (D-Colo.) and Rep. Robert G. Torricelli (D-N.J.) have drafted bills to establish a commission to study the possibility of requiring all young people upon graduation from high school to do some kind of national service, ranging from serving in the Army to insulating the homes of impoverished elderly. The commission would make its recommendation within 15 months.

Torricelli asked Weinberger and Vessey for their views on national service at a hearing of the House Foreign Affairs Committee.

Weinberger said that the all-volunteer military was attracting high-quality people and that he was "constitutionally opposed to fixing something that is not broken."

Vessey said "universal service may have a value to the country, but its value is not to the Department of Defense."

He said the armed services have increased the proportion of career officers from 25 percent to 50 percent since the draft ended in 1973. The higher experience level, he said, has improved performance in such combat skills as bombing and tank gunnery.

Torricelli agreed that volunteers often make more motivated soldiers than draftees but said his bill would preserve that spirit by offering military service as one of several options. Hart and Torricelli intend to introduce their nearly identical bills next week.