Incomprehensible behavior of Netwatch and a script that imitates it.

RouterOS Scripting
2

search tag # rextended check host status

evidenced errors on your script:
:global vdsdown
:local host 192.168.1.252 # better put the editable things up:local checkvds [/ping $host count=3]

:if ($checkvds = 0) do={  # $ must used for call variables:if ($vdsdown != true) do={
	/log warning "Host $host is offline"; # useless ";" :set $vdsdown true # $ must not be used on set}
} else={
	:if ($vdsdown != false = true) do={ # better != false instead of =true/log info "Host $host is online"; # useless ";" :set $vdsdown false # $ must not be used on set}
}

My revised version:

:local host 192.168.0.101

:global vdsdown
:global vdsstatus

# for test "warning unstable" set the count to 2
:local checkvds [/ping $host count=3]

:if ($checkvds = 0) do={
    :if ($vdsstatus != "offline") do={ /log error "Host $host change status from $vdsstatus to offline" }
    :set vdsstatus "offline"
    :set vdsdown true
}

:if (($checkvds > 0) && ($checkvds < 3)) do={
    :if ($vdsstatus != "unstable") do={ /log warning "Host $host change status from $vdsstatus to unstable" }
    :set vdsstatus "unstable"
    :set vdsdown false
}

:if ($checkvds = 3) do={
    :if ($vdsstatus != "online") do={ /log info "Host $host change status from $vdsstatus to online" }
    :set vdsstatus "online"
    :set vdsdown false
}