The waistcoat is finished!
I ended up not using the back belt, it just did not work for me.
When attaching the lining I took Cidell's http://missceliespants.blogspot.com/2008/05/i-sewed-myself-into-corner.html into account and followed her advice on lining a sleeveless garment.
On a different note, I had a little help from my DD who did some handsewing and measuring for me :)
My overall conclusion about this pattern – it is drafted beautifully and allows for a number of modifications, including an easy change of the lapel shape. If I find the right fabric, I will nake it again with the armhole frills.
I am keeping this pattern and maybe will use it as a starting point for drafting a regular waistcoat (without peplum), if I ever need one.