As long as it is not at (or below) firmware 3.55, you wont be able to install a CFW anyways.
A PS Vita with firmware 1.xx, 2.xx or 3.0x can not access the ps vita psn store. You can try to access the psn store from WITHIN TN-V, but keep in mind that that psn store is the psp psn store. Since you just want to redeem a code anyways, that should not be a problem.
Alternative: www.sonyentertainmentnetwork.com.
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