Then how should I complete the original sentence?
1. Our years of hard work are all in vain, not to speak of the large amounts of money we have spent.
2. Our years of hard work are all in vain, to say nothing of the large amounts of money we have spent.
3. Our years of hard work, let alone the large amounts of money we have spent, are all in vain.
4. Our years of hard work, not to mention the large amounts of money we have spent, are all in vain.
I don't much like the phrase "not to mention" simply to introduce things that you do
mention. 2. would be my choice from the above, if I had to pick one. 3. is definitely wrong.
And I guess the reason why "let alone" in sentences 2, 3 and 4 cannot be replaced by "not to mention" is that "not to mention" only takes nominal complements as stipulated in some usage books.
No, that's not the reason. Besides that's wrong, as demonstrated by your correct sentence below.
However, how could you justify the following sentence I find, in which "not to mention" precedes a verb?
It’s impossible for me to go to Green’s party tonight. I’ve got to babysit for my sister, not to mention write a book report and prepare for an exam.
This sentence is OK. "not to mention" is used when the extra information is not necessary to make the main sentence true. So, having to babysit for your sister is enough to make it impossible for you to attend the party. The other things, while true, are not necessary to mention as a reason for not going to the party.
The problem I have with the original sentence is that the additional thing it mentions does add something important, which does bear mentioning and has nothing to do with the hard work being in vain.
This would be a good sentence: "The large amount of money we spent, not to mention the all the hard work we did in vain, was enough to destroy the company."