To be honest if i have to choose i would go with LP because is an amazing small city and u can really relax and feel like back in time, Vientaine is quite modern now and has not the same charme of LP.
Then, if u have just 5 days is hard to say because in all north LAOS the road transport is very slow due to the mountains.
VT - LP by VIP bus: 8 to 12 hours for 380 km!!! (but the trip is really scenic!!!)
If u wanna go farther north allow 6-8 hours one way by bus to same of the small villages (the real ones not the turist traps), to Oudomxay and 8-10 hours to the Plan of Jars and Phonsovan in the east.
Then it's up to you but if u wanna see quite everything in LP u can stay for 3 days (Pak Ou Caves, Kuang SI waterfalls and all the remarkable temples and stuff in the city)