Go to the trade section chat and type WTB [Mag Prime] XXXp replacing the XXX with whatever amount of platinum you're willing to pay for it. Since it is vaulted this is currently the only way to get Mag Prime. Your only other option is to wait until she is unvaulted then hunt down the relics that drop her peices. I would not offer more than 120p for her and you may even get lucky enough to get her for cheaper.
You can also try to find someone with the vaulted relics willing to dump them wholesale. If you're playing PC, I'd be willing to see what I've got and even help out if possible.