Im not 100% sure about all of this but:
Are yordles vastaya or vastayashai’rai?
No, im 100% sure of this one.
Why can they travel to the spirit realm?
Magic? i guess....
Are they strong when compared to humans(magic and strength)?
They all have innate magic, so, they are stronger in that regard. As for strengh, if we look at Tristana, Rumble and Poppy, it seems that they can carry heavy objects (The weapon of Trist for example) without efford, so i guess that they are stronger as well. (Also, Kled)
Do all yordles come from bandle city?
As far as i know: No, Gnar is not from Bandle City since he is from ancient times.
What is bandle city?
A city.