As the plan sponsor (employer) of my company's 401(k) plan, can I be held liable for anything that may happen to my participating employee's 401(k) accounts, if I recommend a vendor who would handle an IRA rollover transaction for them?

If you recommended only one then yes if the Part relied on your advice. If you recommend several vendors and instruct the participant to select one after reviewing the materials from all of the vendors there is little risk of liability because the participant made the choice of vendor. By the way what knid of risk are u concerned with?