Resigning after renewing the visa under an unlimited contract

Dear sir/madam,

I work in a company on the mainland. Can I resign after renewing my visa under an unlimited contract or how long should I wait?

Can I take an annual vacation soon after renewing my visa? Can the employer suddenly change the clause in the signed job contract?

Should I pay visa expenses if I resign because my employer had taken my sign to pay for visa expenses if I resign?