Sometimes, you need to change tty in the VM. You can get to tty1 by holding down in the RIGHT control key and pressing F1, and tty7 (where X is probably running) with right-ctrl + F7

You can also send arbitrary key codes with VBoxManage:
VBoxManage controlvm Vivid keyboardputscancode 1d 38 (ctrl + alt combo)

You can look up other codes here <http://www.win.tue.nl/~aeb/linux/kbd/scancodes-1.html#ss1.4>, but the basic list is:

01 (Esc)
02 (1!), 03 (2@), 04 (3#), 05 (4$), 06 (5%E), 07 (6^), 08 (7&), 09 (8*), 0a (9(), 0b (0)), 0c (-_), 0d (=+), 0e (Backspace)
0f (Tab), 10 (Q), 11 (W), 12 (E), 13 (R), 14 (T), 15 (Y), 16 (U), 17 (I), 18 (O), 19 (P), 1a ([{), 1b (]})
1c (Enter)
1d (LCtrl)
1e (A), 1f (S), 20 (D), 21 (F), 22 (G), 23 (H), 24 (J), 25 (K), 26 (L), 27 (;:), 28 ('")
29 (`~)
2a (LShift)
2b (\|), on a 102-key keyboard
2c (Z), 2d (X), 2e (C), 2f (V), 30 (B), 31 (N), 32 (M), 33 (,<), 34 (.>), 35 (/?), 36 (RShift)
37 (Keypad-*) or (*/PrtScn) on a 83/84-key keyboard
38 (LAlt), 39 (Space bar),
3a (CapsLock)
3b (F1), 3c (F2), 3d (F3), 3e (F4), 3f (F5), 40 (F6), 41 (F7), 42 (F8), 43 (F9), 44 (F10)
45 (NumLock)
46 (ScrollLock)
47 (Keypad-7/Home), 48 (Keypad-8/Up), 49 (Keypad-9/PgUp)
4a (Keypad--)
4b (Keypad-4/Left), 4c (Keypad-5), 4d (Keypad-6/Right), 4e (Keypad-+)
4f (Keypad-1/End), 50 (Keypad-2/Down), 51 (Keypad-3/PgDn)
52 (Keypad-0/Ins), 53 (Keypad-./Del)
54 (Alt-SysRq) on a 84+ key keyboard
55 is less common; occurs e.g. as F11 on a Cherry G80-0777 keyboard, as F12 on a Telerate keyboard, as PF1 on a Focus 9000 keyboard, and as FN on an IBM ThinkPad.
56 mostly on non-US keyboards. It is often an unlabelled key to the left or to the right of the left Alt key.
57 (F11), 58 (F12) both on a 101+ key keyboard
59-5a-...-7f are less common. Assignment is essentially random. Scancodes 55-59 occur as F11-F15 on the Cherry G80-0777 keyboard. Scancodes 59-5c occur on the RC930 keyboard. X calls 5d `KEY_Begin'. Scancodes 61-64 occur on a Telerate keyboard. Scancodes 55, 6d, 6f, 73, 74, 77, 78, 79, 7a, 7b, 7c, 7e occur on the Focus 9000 keyboard. Scancodes 65, 67, 69, 6b occur on a Compaq Armada keyboard. Scancodes 66-68, 73 occur on the Cherry G81-3000 keyboard. Scancodes 70, 73, 79, 7b, 7d occur on a Japanese 86/106 keyboard.