I'm ugandan works with United nations. i want to visit china on vacation. How do i get visa?

You mean you are ugandan and work in Uganda?
If so, you need to provide your passport, application form with recent taken photo, your hotel booking confirmation letter, your round trip air tickets, your bank statement deposit in the past 6 months with the amount no less than SHs5,000,000 and recommendation from your organization for application.

I'm a ugandan work in Darfur sudan with United nation peace keeping mission now i'm planning to visit china on vacation and what i entend is to apply it when i come on leave next month so that is my query about this issue.

If so, you can apply for your Tourist L in China Embassy in Sudan.
You need to provide your passport, application form with recent taken photo, your proof of legal stay in Sudan, your air tickets to China and hotel booking confirmation in China for applying.