Drogo, it seems that you're not utilizing "mono-izer" plugins for your mono tracks? If you do wanna keep them mono, then you need to check out the "question mark" icon on the LADSPA plugin browser from the 'tools' menu. From there you can go to stereo-mono or mono-stereo with the "MS-Stereo" or vise versa. Try if you can keep those tracks mono in the export. Do some tweaks with the audio tracks there. You can also check the "surround matrix encoder" in the same menu. These are some advanced mixing techniques present within lmms.
Use (with caution, as vsts can cause a project loading failure) 'deepanpot' panning vst for panning your tracks either left or right. You can make 2 copies of your vocals, use 'deepanpot' on both of them. Set them to full left and full right so that if there were to be a "stero-izing", then it wouldn't be much different from one or the other. This was used in other DAWs and works perfect for adding double stero effect on stero tracks.
Why would you want to keep your vocals mono? It has to come out through both speakers if you have a stereo setup. So it works out its algorithm to do just that.
May God bless and may peace be with you.
