DXM is a pretty strong serotonin-norepinephrine reuptake inhibitor (SNRI), it's kind of unique among other dissociatives in that aspect. Mixing DXM with stimulants like amphetamine can cause an excessive amount of norepinephrine building up causing "norepinephrine syndrome" also known as a hypertensive crisis. This results in very high blood pressure, agitation, anxiety, risk of stroke and heart attack, and so on and so forth.
I would say you are right on the edge of tolerable dosing. 20mg of Vyvanase is a relatively small dose (and it's time released), but on the other hand 700mg of DXM is a fairly big dose. And you really have to consider both drugs will potentiate each other a bit too. DXM is pretty rough on its own, I don't think having another chemical raising your blood pressure even further would help.
Me personally, I would hold off, just because you can't tell a priori whether or not you'll have a bad response. It's unlikely, but possible. But if this is your one opportunity of a lifetime, proceeding cautiously and letting people know you're mixing drugs (just in case anything goes wrong, it's not a total mystery what you've ingested) is an option too.
Drinking some citrus juice (1-2 liters) and urinating can help flush some of the amphetamine out in your urine, if you have a little bit of forewarning this is a good idea. Don't go overboard and make yourself sick, but just note a reasonable amount of acidic juice ingested will decrease the half life of amphetamine in your body. You can kill 2 birds with 1 stone by drinking grapefruit juice to inhibit the enzymes that break down DXM.
3-MeO-PCP is a straight up NMDA antagonist (pure dissociative), so it is more suited to mixing.
