I hear you... working with vocalists is a unique adventure in and of itself. Finding the right ones that have good chemistry with you, and have a style that works with your work, and well... good cross-promotion opportunity, are hard to dial in.
I work very similarly. In my dance pieces, I almost always write with some sort of vocal sound in mind. So I encourage you to stay with what is resonating with your heart and creativity. Don't give up!
I've been blessed to work with vocalists Christina Novelli, Kirsty Hawkshaw, and Sarah Russell, etc. but I still consider myself indie and small time. I've learned so much in the process, and I have a few strategies and patterns to share, for what its worth.
Endless & Persistent "Cold Calling".Keep aiming high - for the big names you like. Message them on their facebook accounts. Email their managers. Prepare yourself for
tons of rejections, and move on. If your music is good, someone will eventually accept! Even if you are after a specific person, this process is still necessary, because it will build your portfolio.
Snowball EffectNow that you've got a project going on with a more well known vocalist, your cold calling gets easier. You have some names to throw out there with your great music. Vocalists can see that other great vocalists have worked with you.
Pay ThemTip them even. Put the money into it that your project deserves. When asking vocalists to work with you, be upfront about asking them what their rates are, etc. Even if a friend is volunteering to help you out with some vocals, strongly consider giving them some money for it. Even just $50. $20? Something that says, "this song is important to me, and I'm serious about it." It very much will benefit you as an artist to feel yourself investing.
Use Work For HireThe big timers are going to want royalties and more money in advance for the project, so temper your first big one with some work for hires, which will be a lot cheaper, and shouldn't cost you royalties. Check out sites like
https://soundbetter.com/ and find an amazing vocalist on there (they really are awesome...) for $200-300 or so. Use this to fill up your portfolio more with vocalists, so you can finish more songs with them, gaining more experience at working relationally and creatively.
When it comes to the creative process, I'm curious to know more of where you are coming from, because obviously everyone's approach is different, especially when collaborating with another artist, right?

Do you like to be involved in the melody and lyric writing? More hands off? Somewhere in the middle?
Just some of my initial thoughts. Hope some of this is helpful!