The first thing to consider is that song structures for vocalists are going to be very different to normal electronic track structures. They may, in fact, even be actual songs! This is a far cry from the standard dance arrangement which usually goes something like 'intro, beats, breakdown, beats, outro'. Instead, you'll want to be thinking more about verses and choruses. Verses and choruses don't necessarily mean you need to be writing big soaring catchy pop melodies or anything like that. Listen to many of the big RnB or hip-hop tracks out there and you'll notice that the backing tracks often don't change a lot from the verse to the chorus - sometimes it's as subtle as a couple of extra percussion sounds or a pad line - but the point here is that it's sectional and pretty straightforward. Your author once sent a lovingly crafted backing track to a well known grime crew for a vocal - lovingly crafted to the extent that it had all sorts of interesting diversions, 4-bar bridges, little switch-ups and so on, and got back an email that basically said 'sorry, we didn't understand the structure, so here are 3 verses and 3 choruses'.
Equally, since the main thing is the vocal, you should be leaving plenty of space for it in the arrangement. A super-busy percussion section, a huge lead synth, a massive 'filth' bassline - all of these are largely incompatible with a full vocal. That's not to say they have no place in your track - you just need to be careful of having things competing for space. You could try having a very simple chorus vocal which could sit on top of a filthy bassline, and then drop out the bassline (or Low-pass filter it) to focus on the vocal for the verses, for instance. But be aware that a lot of those little details which in an instrumental tune add a touch of interest for the listener, can simply distract from a full vocal.
When writing synth lines and melodies then, don't be afraid to put them in your original idea, but with a view to reducing or reworking them later. They can often give a singer ideas for melodies or harmonies - but then when you record the vocal, they may be masked by the very vocal they inspired. That's fine - don't try to mash everything into the mixdown all front and centre, you can chop them so that they simply accent or answer certain vocal phrases.
The summary of all this is that you should be thinking about the singer and the vocal they will eventually provide, all the way through the writing process. Writing a full track and hoping to graft a vocal on at the end will usually end in tears - you need to consider what the vocalist needs, what the vocal will mean for your arrangement, and how you may have to compromise certain aspects of the track to really make the vocal work. Flexibility is key. If you need to change the track around once you've got the vocal then do it - nothing should be set in stone!
