Hello @dina,
Welcome to the forums! :qiime2:
Both vsearch dereplicate-sequences and cluster-features-de-novo take about the same amount of RAM as the reads after they have been dereplicated or clustered according to the coauthor... So I guess the answer is: try it and see if it works! Sometimes 8gb is enough, and sometimes you need more. 
classify-consensus-vsearch has to load the database into RAM, so you need roughly as much RAM as the database size.
If you would like more detail, Evan written a some great posts about this.
Colin