Para mí, asignación con expresión CASE es un poco confuso (difícil de leer), así que usaría CASE cláusula con INTO @var en cambio:
BEGIN
CASE
WHEN vtable = 'daily_operation' THEN
SELECT start_time INTO @result FROM daily_operation WHERE (client_id = vclient_id AND user_id = vuser_id AND id = vid AND edition_id = vedition_id) LIMIT 1;
WHEN vtable = 'monthly_operation' THEN
SELECT start_time INTO @result FROM monthly_operation WHERE (client_id = vclient_id AND user_id = vuser_id AND id = vid AND edition_id = vedition_id) LIMIT 1;
END CASE;
RETURN @result;
END
Tenga en cuenta que no necesita DECLARAR @session_variables utilizadas en SP.