vocals disappear

What are you actually doing?
As you can't send a cwp-file to an iPad or phone, the song is exported first in some form. Which, what, how?
Producing final mixes that transfer well to all soundsystems is the most difficult thing in home recording.

This sounds like you're only getting one channel of the stereo mix playing. 
 
My jack converter (1/4" to 1/8th) sometimes does that when I plug in the cans and that's exactly what it sounds like..... do a mono mix and test that the same way.

Interesting topic as I am just finishing an album so uploaded a few of the tracks to Sound clicks. 
I told a friend about the album and got him to go to the web page and we listened to few and I was impressed they they actually translated perfectly on his iphone. I was unaware that they are mono too. In my old age I've gotten pretty good at this translating game ,, pat on back if my arm wasn't so sore... 
There's a MONO button on your master buss,,, use it. 

Re-reading the OP and the answers I'm also sure it's a phase issue due to mono playback. I just wanted a confirmation that the original exported version is checked to be OK.

And I didn't mean export in mono, I meant to "check"you mix in mono, Then export in stereo. 

That explains it. You don't normally add reverb in series to your signal. You add it in parallel. 
 
I'll give you an example of how I set up a mix. For simplicity I will not use any sub mix buses. 
A sub mix buss would be put between your tracks and the master. 
In the Top Pane: 
will include the Audio,MIDI and soft synth tracks. The Audio and Soft synths Audio are all output to the Master Buss.
 
In the bottom pane: 
A Master Buss - output to interface
A Reverb Buss - output to master
A Delay Buss - Output to master 
And there could be one more for guitar effects like Chorus or Leslie. 
and you might also have a metronome and headphone buss (that have nothing to do with your mix) 
 
Tracks and buses all have an effect bin. 
I will put  EQ or compression in the track bins. I realize some would also put guitar effects here like distortion, I record those sounds live, so each to there own in this matter there is no right or wrong.  All other effects will go in a Buss effect bin. And only one Effect per buss. 
 
I will put the LP 64 Multi band in the Master bus bin. This is in bypass mode until final export to avoid tracking latency issues. . 
 
All your tracks have the ability to insert an effect send to any of the inserted buses. You use this to send part of your tracks signal in parallel to that effect. 
This also has all your tracks sharing the same effects which makes for a more natural sounds-cape. 
People are not used to hearing sound with 6 different spacial "rooms" They want to be in one place. 
 
Of course you could put an effect in the track bin and use the wet/dry to mix, but it is so much better when using console view to set it up like I do. All your controls are easy to see and reach, and you can solo the effect buses or mute them.