Hoping that I understood your question correctly, you are trying to dock a button to the left of the ToolStrip Control as shown in the image below.
To achieve this you need to first set the Dock property of both the controls to Left
To have the button control as the first left docked control, you can achieve this by using the Document Outline window which is opened using the View > Other Windows > Document Outline menu and then setting the dock priority by dragging the button below the ToolStrip as shown in the image above.
To support multiple buttons you could replace the single button in the above sample, with a panel containing the needed buttons.
Also in order to avoid reinventing the wheel, you can have a look at the DockPanel Suite project that is available for free on SourceForge.
EDIT: As requested in comment.
You could link all the buttons that you want to support vertical text to the paint event as shown below
private void VerticalButtonTextEvent(object sender, PaintEventArgs e)
{
Button button = sender as Button;
if (button == null) return;
Graphics g = e.Graphics;
g.FillRectangle(SystemBrushes.Control, button.ClientRectangle);
using (Font f = new Font("Times New Roman", 8))
{
SizeF szF = g.MeasureString(button.Text, f);
g.TranslateTransform(
(float) ((Button) sender).ClientRectangle.Width/(float) 2 + szF.Height/(float) 2,
(float) ((Button) sender).ClientRectangle.Height/(float) 2 -
(float) szF.Width/(float) 2);
g.RotateTransform(90);
g.DrawString(button.Text, f, Brushes.Black, 0, 0);
}
}