Firstly, P.vivax does invade human RBCs but it prefers the very young RBCs called 'reticulocytes' that comprise less than 5% of the human blood and that leads to the answer to the second question. The in vitro culture of P. vivax is faced with the problem of getting sufficient reticulocytes from blood and then replenishing the reticulocytes in culture because these young RBCs become older RBCs in 2-3 days.
P. vivax may invade humans. What is very interesting malaria infections were diagnosed in Poland (quite different climate to Africa or India) until 1963. Two Plasmodium species were responsible for those infections : Plasmodium vivax hibernans and P. vivax vivax. Definitive hosts were: Anopheles messae, A. typicus, A. bifurcatus and A. atroparvus.
I am agree with Makindi David, I would like to add that P. vivax selectively invades Duffy positive reticulocytes (immature RBC). More details you may also find in my recent opinion article attached.
Plasmodium vivax do invade humans RBC and the kind of RBC that it invade is young RBC. If you look under the microscope it is more larger than the other RBC.