PNG reached 250 for two in only 42.5 overs.
Vala hit eight fours and six sixes in his 105 not out; Siaka needed only 83 balls for his ton.
In 1996, Kenya beat the West Indies of Lara, Richardson, Ambrose and Walsh by 73 runs at the World Cup. They have appeared at the past five World Cups; making this one might be a struggle.
Hong Kong hit 263 for seven at Queenstown Events Centre and despite a century from Durban-born Peter Mommsen, the Scots, coached by former England batsman Paul Collingwood, came up 17 runs short.
The Netherlands eased past Uganda, in Mt Maunganui, with half centuries from Wesley Barresi and Christchurch-born captain Peter Borren; while the UAE produced the opening day's biggest total, 297 for eight, with Shaiman Anwar hitting 109, one of five hundreds in the day.
Tomorrow Canada make their debut against the UAE at Rangiora, while the Dutch play Namibia at Mt Maunganui.
First day's play
Mt Maunganui: Uganda 151 (46.1 overs) (R. Mukasa 44, A. Mutyaguba 36) lost to the Netherlands 155-3 (27 overs) (W. Barresi 61 not out, P. Borren 50) by seven wickets.
New Plymouth: Kenya 249-9 (50 overs) (C.Obuya 106, T. Odoyo 42; R. Haoda 4-50) lost to Papua New Guinea 250-2 (42.5 overs) (L. Siaka 103, A. Vala 105 not out) by eight wickets.
Rangiora: United Arab Emirates 297-8 (50 overs) (S. Anwar 109, S. Patil 74; B. Regmi 3-43) bt Nepal 195 (48.1 overs) (S. Vesawkar 54, S. Gauchan 37 not out, B. Bhandari 37; K. Shazad 3-46) by 102 runs.
Queenstown: Hong Kong 263-7 (50 overs) I. Ahmed 75, Y. Azfal 42; I. Wardlow 3-57) bt Scotland 246 (48.4) (P. Mommsen 118, M. Machan 34; N. Ahmed 4-37, H. Amjad 3-53) by 17 runs.
Points
Pool A: UAE 2, Hong Kong 2, Scotland, Nepal and Canada 0.
Group B: Netherlands 2, Papua New Guinea 2, Kenya, Namibia, Uganda 0.
