1971 , 198 slant vacuum hose

What hose goes down to the side of the block near fuel pump & coil ? When it used to be hooked to my single barrel carter B&b all was find other than my carb was junk .
I converted to 2bbl yesterday and things are amazingly better . But when I hook that hose up it sucks oil into intake and I blow blue smoke out tail pipe . Why is this ? Is the 2bbl just have a much higher/ lower / deeper vacuum and sucking it up ?

disconnected no oil comes up the hose at all . I just have it venting under hood and nothing comes out and I plugged the port at carb .

why or how important is that hose ?