How do I force a register to be linewise before I put it in Vim?
:call setreg('a', @a, 'V')
When you paste from a register, Vim uses that register's stored type: characterwise, linewise, or blockwise.
Tag: setreg
How do I convert a register's type between characterwise and linewise?
:call setreg('a', @a, 'l')
Registers in Vim have a type — characterwise, linewise, or blockwise — that affects how their contents are pasted.
How do you set a register programmatically in Vim?
:call setreg('a', 'text')
Use setreg() to set register contents from Vimscript.