In reply to @mosullivan93:matrix.org
The problem seems to be that after you detect you're on the line starting with "(..." you then try to pass in the vector itself to the read function

thx for the tips I finally was able to parse my file!
The big things were the casting of Vector{UInt8} and the function reinterpret

Here is the finally snippet I ended up with:

function read_bt_paran(file)
    fh = open(file)

    paran_start = false
    paran_end = false
    paran_end_maybe = false
    bt_paran_str = ""
    num_cells = 0
    format_str = ""

    while !eof(fh)
        line = readline(fh, keep=true)
        if occursin("format", line)
            split_line = split(line)
            format_str = split(split_line[2], ";")[1]
        end
        if occursin("nCells", line)
            split_line = split(line)
            num_cells_str = split(split_line[3], ":")[2]
            num_cells = parse(Int32, num_cells_str)
        end
        if occursin(")\n", line)
            if format_str == "ascii"
                paran_end = true
            end
            paran_end_maybe = true
        end
        if line == "\n" && !paran_end && paran_end_maybe
            paran_end = true
            bt_paran_str = bt_paran_str[1:end-2]
        end
        if paran_start && !paran_end
            bt_paran_str = bt_paran_str * line
        end
        if occursin("(", line) && !paran_start
            paran_start = true
            if format_str == "binary"
                bt_paran_str = bt_paran_str * line[2:end]
            end
        end
    end

    close(fh)

    bt_paran_vec = Vector{Int32}()
    if format_str == "ascii"
        bt_paran_vec = parse.(Int32, split(bt_paran_str))
    end
    if format_str == "binary"
        bt_paran_uint8_vec = Vector{UInt8}(bt_paran_str)
        bt_paran_vec = reinterpret(Int32, bt_paran_uint8_vec)
    end

    bt_paran_vec = bt_paran_vec .+ 1

    return bt_paran_vec, num_cells
end

Def one of the weirdest files I've had to parse haha