Metagenomics: Identifying most common sequences
According to the VSEARCH docs, since you have specified --sizeout your abundances have been written into the FASTA headers:

--sizeout
Take into account the abundance annotations present in the input fasta file (search for the pattern ’[&gt;;]size=integer[;]’ in sequence headers). That option is active by default when rereplicating.
Add abundance annotations to the output fasta file (add the pattern ’;size=integer;’ to sequence headers). If --sizein is specified, each unique sequence receives a new abun- dance value corresponding to its total abundance (sum of the abundances of its occur- rences). If --sizein is not specified, input abundances are set to 1, and each unique sequence receives a new abundance value corresponding to its number of occurrences in the input file.