What is the meaning of "v'Lo Yihyeh Lachem"?
Radak, Malbim: This refers to everything mentioned above - they will not have a house, seeded field or vineyard, even via a gift or purchase. Malbim - Had he said only "Lo Sivnu" and "Lo Sita'u", this would imply that they may buy what others built or planted.
Where will their tents be?
Rashi: In Midbaros.
How will this cause them to live many days?
Radak: Houses, fields and vineyard keep a person in his place. If there will be hunger or sword, he will die prematurely from worry. When he will not be in his place, as long as he sees good in this place he will dwell there, and when evil comes he will go elsewhere, where it will be good. This is why he commanded that they dwell in tents and travel from place to place, according to what they see is good, instead of having a house, field and vineyard. We find that the Avos traveled from place to place due to famine.
Why does it say "Asher Atem Garim Sham"?
Rashi: They did not receive a portion of the land, for they descended from converts (Yisro).