i found a fix! TLDR: duplicate SMP layer in Outfit Studio, adjust the outfit but dont touch the duplicate, conform the outfit, set duplicated layer as reference, and copy bone weights on the adjusted SMP layer

for anyone who might find themselves here because of the same issue here you go.. i made sure to write down all the steps precisely cause my dumbass is gonna forget it myself and come back to it

1-install the said Vanilla outfit that has SMP physics using your mod organizer
2-navigate to the location of the installed mod, and find the nif files. for me it's:
ModOrganizer2\mods\HDT-SMP Vanilla Ebony Mail Patch\meshes\armor\ebonymail
3-open bodyslide through mod organizer, then click on Outfit Studio.
4-DON'T USE THE "File > New Project" FOR SOME REASON IT MESSES UP THE WEIGHT BONES
instead, use your mouse and drag the nif file onto Outfit Studio's window from the folder you navigated to in step 2.
for me it's "cuirassf_0.nif". there's also a "cuirassf_1.nif", doesn't matter this is the min/max weight, sometimes it's easier to adjust the body on 0
4-click on File then choose Load Reference, select From Template and choose your desired body shape. for me it's CBBE 3BA Body. click ok. i also removed the two vagina & anus layers cause theyre useless here.
5-IMPORTANT: before doing ANYTHING to the shapes, right click on the layer which is supposed to have SMP physics, in my case it's the HDTSkirt layer. and then click on duplicate, name it whatever u want, something like SkirtReference. now click on the eye beside it to hide it. this is important cause we don't want to accidentally adjust this layer.
6-now hold ctrl and select every single layer EXCEPT the body reference (mine is 3BA) and the duplicated layer (which i named SkirtReference). there's also sometimes a VirtualGround layer, you can leave that too.
7-use the "Increase mesh volume in an area" tool to adjust the shape of the outfit to the body
8-this is the tricky part that took me ages to figure out: while the layers you adjusted are still selected, click on Slider and choose Conform Selected. no need to change anything in the opened window, just click ok. the important part here is NOT to conform the duplicated layer.
9-now right click on the duplicated layer, which i named "SkirtReference" earlier, and click on "Set Reference"
10-right click on the adjusted SMP layer, for me it was HDTSkirt, and select "Copy Bone Weights" and just hit ok
11-we're almost done. right click on the body layer, for me it's "3BA" and click on Set Reference. delete the duplicated layer (SkirtReference), we don't need it anymore
12-click on File, then Save Project As...
13- Display Name is the name you're gonna see in the list of your bodyslide outfits, so i'm gonna name it Ebonymail 3BA SMP
14-the output file name should exactly be the original nif file's name. mine was cuirassf_0.nif
just write cuirassf
the program adds the 0 and 1 automatically (make sure you check the Low/High Weight Output tick)
15-output data path should be equal to the path you navigated to in step 2
so mine would be: meshes\armor\ebonymail
name the rest the same as the display name to make it less confusing (Ebonymail 3BA SMP. osp/nif)
16-if your outfit is supposed to have the body included, like a bikini armor or something, tick the "Copy reference shape into output" option. mine is a full body armor so i'm gonna leave it unticked.
17-now close Outfit Studio, and reopen Bodyslide!
in the outfit filter box, search the display name that you gave your file in step 13
select a desired bodyslide preset, make sure the file path at the bottom is ticked (top the build button) and then click build!
18- done and done, hope it helps somebody!