HK -> Beijing -> Vietnam

Hi,

I have a question. I am Portuguese and I will leave Hong Kong after spending a few days there and then spend 4 days in Beijing with plans of going to the Great Wall and surrounding areas and then leave for Vietnam. Our flights aren't technically layovers since they are two one-way tickets but both are confirmed and we have seat assignments. Is that ok for 144 visa? Can the flights be separate? Also, how far can we go from Beijing? Is it like a municipal city limit?

Thanks