Parameters take input before arguments. What you should do instead is add a parameter that accepts an array, and make it the first position parameter. ex:
param(
[Parameter(Position = 0)]
[string[]]$Hosts,
[string]$VLAN
)
foreach ($i in $Hosts)
{
Do-Stuff $i
}
Then call it like:
.\script.ps1 host1, host2, host3 -VLAN 2
Notice the comma between the values. This collects them in an array