Sorry in advance for the long winded answer.
I was having this same dilemma this past year. I've made a lot of changes to my optics this year and learned a lot. I think a lot of it depends on where, what, and how you're hunting... There is not 1 best optics setup for every situation and every kind of terrain.
I have a buddy that uses 18s almost exclusively on his OTC elk in Colorado where he is just looking for pretty much a legal bull. This is primarily because the country he hunts lends itself very well to this kind of glassing. This last season he said he packed his swaro spotter the first day then left it at home, he can usually tell if they are a bull or not with just the 18s.
However, there are other places out there (like where I elk hunt) that high power binos would be nearly useless, flat country with no good vantage points with thick oak brush... 18s would probably get left in the truck on this hunt..
Almost all of my time is spent behind my 8x42s and a tripod, I only pull out the spotter if I need to check something. Maybe that would change if I had 15s or 18s.. But I think I would still need the spotter for sizing stuff up once I spot it.
My suggestion to you in order of importance is:
1: as suggested above, if you don't already have one, get a tripod and start glassing off of that, (slik tripod with a benro S2 head is a good setup that doesn't break the bank). You'll need it to run high power binos or a spotter, but I use it more with my 8s than anything.
2: Sell your vortex binos and upgrade to the absolute best set of 8s or 10s you can afford. (Watch the classifieds, I scored on a used set of alpha glass this last year, they BLOW my old vortex vipers out of the water.. its night and day glassing behind them compared to my vipers). I 100% believe that this is where you should invest your money first, you will easily spend >90% of your time behind these. Wish I had got into high end glass sooner.
3: Then at this point I would probably buy a spotting scope.. do your research. I recently sold my vortex Razor hd 85mm and bought a Kowa 66mm spotter. I really only use the spotter once I've spotted something to check it out, and the kowa weighs about half of what the vortex did so lugging it around all day isn't as much of a burden. This spotter does everything I need it to do, although without question my buddies swaro is hands down nicer, the kowa is fairly light and gets the job done.
4: At this point I would suggest you demo a set of high power mavens and use them on a hunt or scouting trip and see for yourself if you like them enough to justify dropping the extra money.. I did this recently on a 3rd season mule deer hunt. Maven made it super easy and painless, Just floated the money on a credit card, used the binos for 2 weeks then returned and got refunded all but the shipping.
They didn't have any of their new 15s or 18s available when I did this, so I demoed a set of the B4 12x56s, they were pretty nice, but they didn't offer much advantage for me over my Leica 8x42s. I went back and fourth a lot, and for me 12s do not justify spending the extra money, In fact I preferred glassing behind the 8's. I will likely do the same thing next year with a set of their 15s or 18s (or maybe both) and see if those offer me enough that I can justify dropping the $$..
I will say that packing 8s or 10s on your chest, plus heavy high power binos, a spotter, and a tripod all in your pack adds up fast, and is a pain in the a$$.. I think that the high power binos are the most likely to get left at home, unless the particular terrain or circumstances lend themselves very well to the high power binos like the example I gave earlier about my buddy.
I guess this was a very long winded way to say this.. Good 8s or 10s on a tripod and a spotter is a pretty deadly setup, you can spot A LOT of game with this setup. I think the this should be your starting point, then maybe save up and expand on this with a set of high power binos after, I think of high power binos as supplemental or circumstantial depending on the hunt.