Issue installing VIB on ESXi 6.5U1
Solution 1:
Use the absolute path to your zip file. It's even documented:
esxcli --server=server_name software sources vib list --depot=absolute_path_to_depot_zip_file
In your case:
esxcli software vib install -d "/vmfs/volumes/datastore1/ISOs/OM-SrvAdmin-Dell-Web-9.1.0-3523.VIB-ESX60i_A00 (2).zip"
Note the missing / at the start of your path.