the best way to monetize a free VPS
is to run traffic exchange programmes and either sell the accounts
(a decent Hitleap account with 100k+ minutes can get you 10-50$ each,for example)
or use the visits to convert them into cash at some PTP sites or sell traffic in bulk
for more info you would want to research yourself
google PTP and TE etc.
I could give some reflinks if you pm me here too
p.s. 20 VPS running 24/7,provided their TOS allow TE usage,could bring you extra 100-200$ a month
but do not expect to become a millionaire,obviously