The employee's 401(k) benefits will:

Under USERRA, employees' vesting in pension plans, including 401(k), must continue during military service as if they had remained continuously employed. No break in service occurs for vesting purposes.

Official Extract:

"An employee’s military leave must not result in a loss of pension or retirement plan benefits, including vesting, as if continuously employed."

(Source: HRCI PHR Content Outline 2024–2025, Employee and Labor Relations Section, Pension and Retirement Protections under USERRA)