I have an idea, although it isn`t exactly what you asked for...
If this cologne really is that old, I really (!!!!!!) wouldn`t waste it in soap! (pleaseplease
please don`t...)
People pay good money to get their hands on true vintage bottles, even if you don`t have the box it came in (double score if you do)
I love vintage perfumes and I have several of them (not bought, just saved them from when I was young, they are worth big money now.)
Yours do too:
HERE
HERE
HERE
Instead, sell it on ebay etc - get money from it. Then, buy soap supplies, like fragrances or essential oils etc?
Last note: What if you
do use it in the soap, it ends up smell amazing, but the scent is impossible to get a hold of ever again. So you have wonderfull soap that smells divine and you savour every bit of it, because it reminds you of your mom. But it will be a soap you won`t be able to duplicate again (unless you have to pay $40 for a half empty bottle that smells off)
Just wanted to throw that out there : )