1.d4 Nf6 2.c4 c6 is best
After 1.d4 Nf6 2.c4 I think 2...c6 is the only reasonable move order to try to get Slav defense. I would consider this not ideal as you allow 3.Bf4 which gives White quite an interesting version of the London System, as a c6-d5 pawn structure isn't ideal vs the London when black is usually trying to exploit Whites slightly weakened dark squares on the queenside with an early ...c5. However if you consider 3.Bf4 to not be a problem then I don't see any way White can exploit this move order, as any other logical third White move can be met by 3...d5.
Other second moves I would consider much worse if the goal is to get a typical Slav defense. 2...e6, besides allowing an exchange QGD after any move such as 3.Nc3/f3 d5 followed by 4.cxd5. Also allows White to go into a Catalan with 3.g3 which could be quite annoying for a Slav player as the early commitment to e6 cuts out a lot of Blacks best options against 1.d4 d5 2.c4 c6 3.g3.
Finally 1.d4 Nf6 2.c4 d5? allows the simple cxd5, where the top 4 engine moves are not even 3...Nxd5 as that allows 4.Nf3 followed by 5.e4 with large advantage for White. If you want to play for the initiative 3...c6 is an interesting gambit, 3...g6 will simply lead to a version of the Grunfeld which is worse for Black than mainlines.
In summary 1.d4 Nf6 2.c4 c6 is a fine way to transpose to the Slav which doesn't allow white many reasonable extra options. If you can work out something you like again 3.Bf4 then the worse that will happen is a transposition back to a Slav mainline after 3.Nc3, e3, Nf3 etc.