I agree with Phill on this…
Definitely, do not stretch the vinyl
* Apply the text as you would normally and slit against the edge of each plank. “allow it to roll over the edge slightly though. so you do not have an exposed edge”
* Then drop the trimmed vinyl down onto the face of the lower plank, apply it, trim against its edge and repeat.
* Once you have completed it, stand back and if you feel that you can see the underside of the “returns”
Simply apply strips of 20mm black vinyl and trim to meet the letter face.
Be sure to apply heat to the face of the vinyl and rub down firmly with a wrap glove or cotton rag to conform with the texture of the wood.
The Logo will be a bit more difficult, but the same process should apply.
However, those 20mm strips will have to be cut from some printed parts of the logo to keep the colours the same.
I am saying 20mm because you said the returns are 20mm, It is probably best to make it something like 23 or 25mm to allow for the uneven thickness of wood in general.
NOTE:
you really should be fitting the vinyl to a gloss surface.
If this is old wood and matt-faced, textured. you could be opening a can of worms for yourself.
If the jobs pay well, it might be better to use a high-tack, low-energy surface type of vinyl/adhesive.
Keep in mind, that vinyl will only last as long as the paint surface holding it.